use gi.invalid_obj instead of cg.zeroobj

cg.zeroobj was originally added (under its previous unprefixed name)
for providing a one-line way to zero out the fields of a struct obj.

    struct obj tempobj;
    tempobj = cg.zeroobj;

    initfn(struct obj *otmp)
    {
        if (otmp)
            *otmp = cg.zeroobj;
    }

More recently, the address of cg.zeroobj began to be used as a return
flag to indicate some things, but the 'const struct obj zeroobj' wasn't
an ideal fit for the purpose and required a number of casts, including
casting away const.

Provide a better fitting variable (gi.invalid_obj) and eliminate a
number of casts.

include/decl.h

@@ -467,6 +467,9 @@ struct instance_globals_i {
     /* sp_lev.c */
     boolean in_mk_themerooms;
 
+    /* new */
+    struct obj invalid_obj;
+
     boolean havestate;
     unsigned long magic; /* validate that structure layout is preserved */
 };

src/apply.c

@@ -2564,7 +2564,7 @@ use_grease(struct obj *obj)
         consume_obj_charge(obj, TRUE);
 
         oldglib = (int) (Glib & TIMEOUT);
-        if (otmp != &cg.zeroobj) {
+        if (otmp != &gi.invalid_obj) {
             You("cover %s with a thick layer of grease.", yname(otmp));
             otmp->greased = 1;
             if (obj->cursed && !nohands(gy.youmonst.data)) {

src/decl.c

@@ -440,6 +440,8 @@ const struct instance_globals_i g_init_i = {
     UNDEFINED_PTR, /* id_map */
     /* sp_lev.c */
     FALSE, /* in_mk_themerooms */
+    DUMMY,  /* invalid_obj */
+
     TRUE, /* havestate*/
     IVMAGIC  /* i_magic to validate that structure layout has been preserved */
 };

src/eat.c

@@ -2720,7 +2720,7 @@ doeat(void)
     }
 
     /* from floorfood(), &zeroobj means iron bars at current spot */
-    if (otmp == &cg.zeroobj) {
+    if (otmp == &gi.invalid_obj) {
         /* hero in metallivore form is eating [diggable] iron bars
            at current location so skip the other assorted checks;
            operates as if digging rather than via the eat occupation */
@@ -3507,7 +3507,7 @@ floorfood(
                 c = yn_function(qbuf, ynqchars, 'n', TRUE);
             }
             if (c == 'y')
-                return (struct obj *) &cg.zeroobj; /* csst away 'const' */
+                return (struct obj *) &gi.invalid_obj; /* csst away 'const' */
             else if (c == 'q')
                 return (struct obj *) 0;
             ++getobj_else;

src/engrave.c

@@ -532,7 +532,7 @@ doengrave(void)
     if (!otmp) /* otmp == cg.zeroobj if fingers */
         return ECMD_CANCEL;
 
-    if (otmp == &cg.zeroobj) {
+    if (otmp == &gi.invalid_obj) {
         Strcat(strcpy(fbuf, "your "), body_part(FINGERTIP));
         writer = fbuf;
     } else {
@@ -563,7 +563,7 @@ doengrave(void)
         return ECMD_OK;
     }
     if (IS_GRAVE(levl[u.ux][u.uy].typ)) {
-        if (otmp == &cg.zeroobj) { /* using only finger */
+        if (otmp == &gi.invalid_obj) { /* using only finger */
             You("would only make a small smudge on the %s.",
                 surface(u.ux, u.uy));
             return ECMD_OK;
@@ -1024,7 +1024,7 @@ doengrave(void)
     }
 
     /* Tell adventurer what is going on */
-    if (otmp != &cg.zeroobj)
+    if (otmp != &gi.invalid_obj)
         You("%s the %s with %s.", everb, eloc, doname(otmp));
     else
         You("%s the %s with your %s.", everb, eloc, body_part(FINGERTIP));
@@ -1131,7 +1131,7 @@ engrave(void)
     }
     /* Stylus might have been taken out of inventory and destroyed somehow.
      * Not safe to dereference stylus until after this. */
-    if (gc.context.engraving.stylus == &cg.zeroobj) { /* bare finger */
+    if (gc.context.engraving.stylus == &gi.invalid_obj) { /* bare finger */
         stylus = (struct obj *) 0;
     } else {
         for (stylus = gi.invent; stylus; stylus = stylus->nobj) {

src/files.c

@@ -3880,7 +3880,7 @@ fopen_wizkit_file(void)
 static void
 wizkit_addinv(struct obj *obj)
 {
-    if (!obj || obj == &cg.zeroobj)
+    if (!obj || obj == &gi.invalid_obj)
         return;
 
     /* subset of starting inventory pre-ID */
@@ -3912,7 +3912,7 @@ proc_wizkit_line(char *buf)
     otmp = readobjnam(buf, (struct obj *) 0);
 
     if (otmp) {
-        if (otmp != &cg.zeroobj)
+        if (otmp != &gi.invalid_obj)
             wizkit_addinv(otmp);
     } else {
         /* .60 limits output line width to 79 chars */

src/fountain.c

@@ -391,7 +391,7 @@ void
 dipfountain(struct obj *obj)
 {
     int er = ER_NOTHING;
-    boolean is_hands = (obj == &cg.zeroobj);
+    boolean is_hands = (obj == &gi.invalid_obj);
 
     if (Levitation) {
         floating_above("fountain");
@@ -713,7 +713,7 @@ dipsink(struct obj *obj)
         if (Glib && (!obj || obj == uarmg))
             Your("%s are still slippery.", fingers_or_gloves(TRUE));
         return;
-    } else if (obj == &cg.zeroobj || obj == uarmg) {
+    } else if (obj == &gi.invalid_obj || obj == uarmg) {
         (void) wash_hands();
         return;
     } else if (obj->oclass != POTION_CLASS) {

src/invent.c

@@ -1527,13 +1527,13 @@ any_obj_ok(struct obj *obj)
  * getobj returns:
  *      struct obj *xxx:        object to do something with.
  *      (struct obj *) 0        error return: no object.
- *      &cg.zeroobj             explicitly no object (as in w-).
+ *      &gi.invalid_obj         explicitly no object (as in w-).
  * The obj_ok callback should not have side effects (apart from
  * abnormal-behavior things like impossible calls); it can be called multiple
  * times on the same object during the execution of this function.
  * Callbacks' argument is either a valid object pointer or a null pointer,
  * which represents the validity of doing that action on HANDS_SYM. getobj
- * won't call it with &cg.zeroobj, so its behavior can be undefined in that
+ * won't call it with &gi.invalid_obj, so its behavior can be undefined in that
  * case.
  */
 struct obj *
@@ -1579,7 +1579,7 @@ getobj(
                     /* check whether the hands/self choice is suitable */
                     v = (*obj_ok)((struct obj *) 0);
                     if (v == GETOBJ_SUGGEST || v == GETOBJ_DOWNPLAY)
-                        otmp = (struct obj *) &cg.zeroobj;
+                        otmp = &gi.invalid_obj;
                 } else {
                     /* there could be more than one match if key is '#';
                        take first one which passes the obj_ok callback */
@@ -1742,7 +1742,7 @@ getobj(
         if (ilet == HANDS_SYM) { /* '-' */
             if (!allownone)
                 mime_action(word);
-            return (allownone ? (struct obj *) &cg.zeroobj : (struct obj *) 0);
+            return (allownone ? &gi.invalid_obj : (struct obj *) 0);
         }
  redo_menu:
         /* since gold is now kept in inventory, we need to do processing for
@@ -1778,7 +1778,7 @@ getobj(
                 continue;
             }
             if (ilet == HANDS_SYM)
-                return (struct obj *) &cg.zeroobj; /* cast away 'const' */
+                return &gi.invalid_obj;
             if (ilet == '\033') {
                 if (flags.verbose)
                     pline1(Never_mind);

src/muse.c

@@ -2886,7 +2886,7 @@ munslime(struct monst *mon, boolean by_you)
             }
         }
         if (t && t->ttyp == FIRE_TRAP)
-            return muse_unslime(mon, (struct obj *) &cg.zeroobj, t, by_you);
+            return muse_unslime(mon, &gi.invalid_obj, t, by_you);
 
     } /* MUSE */
 

src/objnam.c

@@ -3266,7 +3266,7 @@ wizterrainwish(struct _readobjnam_data *d)
                   (trap != MAGIC_PORTAL) ? "" : " to nowhere");
         } else
             pline("Creation of %s failed.", an(tname));
-        return (struct obj *) &cg.zeroobj;
+        return &gi.invalid_obj;
     }
 
     /* furniture and terrain (use at your own risk; can clobber stairs
@@ -3540,7 +3540,7 @@ wizterrainwish(struct _readobjnam_data *d)
     }
     if (madeterrain || badterrain) {
         /* cast 'const' away; caller won't modify this */
-        return (struct obj *) &cg.zeroobj;
+        return &gi.invalid_obj;
     }
 
     return (struct obj *) 0;
@@ -4495,7 +4495,7 @@ readobjnam_postparse3(struct _readobjnam_data *d)
  * Return something wished for.  Specifying a null pointer for
  * the user request string results in a random object.  Otherwise,
  * if asking explicitly for "nothing" (or "nil") return no_wish;
- * if not an object return &cg.zeroobj; if an error (no matching object),
+ * if not an object return &gi.invalid_obj; if an error (no matching object),
  * return null.
  */
 struct obj *
@@ -4960,7 +4960,7 @@ readobjnam(char *bp, struct obj *no_wish)
          || (d.otmp->oartifact && rn2(nartifact_exist()) > 1)) && !wizard) {
         artifact_exists(d.otmp, safe_oname(d.otmp), FALSE, ONAME_NO_FLAGS);
         obfree(d.otmp, (struct obj *) 0);
-        d.otmp = (struct obj *) &cg.zeroobj;
+        d.otmp = (struct obj *) &gi.invalid_obj;
         pline("For a moment, you feel %s in your %s, but it disappears!",
               something, makeplural(body_part(HAND)));
         return d.otmp;

src/potion.c

@@ -2246,7 +2246,7 @@ dodip(void)
     if (inaccessible_equipment(obj, "dip", FALSE))
         return ECMD_OK;
 
-    is_hands = (obj == &cg.zeroobj);
+    is_hands = (obj == &gi.invalid_obj);
     shortestname = (is_hands || is_plural(obj) || pair_of(obj)) ? "them"
                                                                 : "it";
     drink_ok_extra = 0;
@@ -2381,7 +2381,7 @@ potion_dip(struct obj *obj, struct obj *potion)
         pline("That is a potion bottle, not a Klein bottle!");
         return ECMD_OK;
     }
-    if (obj == &cg.zeroobj) {
+    if (obj == &gi.invalid_obj) {
         You("can't fit your %s into the mouth of the bottle!",
             body_part(HAND));
         return ECMD_OK;

src/weapon.c

@@ -509,7 +509,7 @@ select_rwep(struct monst *mtmp)
 
     char mlet = mtmp->data->mlet;
 
-    gp.propellor = (struct obj *) &cg.zeroobj;
+    gp.propellor = &gi.invalid_obj;
     Oselect(EGG);      /* cockatrice egg */
     if (mlet == S_KOP) /* pies are first choice for Kops */
         Oselect(CREAM_PIE);
@@ -568,7 +568,7 @@ select_rwep(struct monst *mtmp)
         }
 
         /* KMH -- This belongs here so darts will work */
-        gp.propellor = (struct obj *) &cg.zeroobj;
+        gp.propellor = &gi.invalid_obj;
 
         prop = objects[rwep[i]].oc_skill;
         if (prop < 0) {
@@ -593,7 +593,7 @@ select_rwep(struct monst *mtmp)
                 gp.propellor = 0;
         }
         /* propellor = obj, propellor to use
-         * propellor = &cg.zeroobj, doesn't need a propellor
+         * propellor = &gi.invalid_obj, doesn't need a propellor
          * propellor = 0, needed one and didn't have one
          */
         if (gp.propellor != 0) {
@@ -788,7 +788,7 @@ mon_wield_item(struct monst *mon)
                    mon_nam(mon));
         return 0;
     }
-    if (obj && obj != &cg.zeroobj) {
+    if (obj && obj != &gi.invalid_obj) {
         struct obj *mw_tmp = MON_WEP(mon);
 
         if (mw_tmp && mw_tmp->otyp == obj->otyp) {

src/wield.c

@@ -384,7 +384,7 @@ dowield(void)
     }
 
     /* Handle no object, or object in other slot */
-    if (wep == &cg.zeroobj) {
+    if (wep == &gi.invalid_obj) {
         wep = (struct obj *) 0;
     } else if (wep == uswapwep) {
         return doswapweapon();
@@ -516,7 +516,7 @@ doquiver_core(const char *verb) /* "ready" or "fire" */
     if (!newquiver) {
         /* Cancelled */
         return ECMD_CANCEL;
-    } else if (newquiver == &cg.zeroobj) { /* no object */
+    } else if (newquiver == &gi.invalid_obj) { /* no object */
         /* Explicitly nothing */
         if (uquiver) {
             You("now have no ammunition readied.");

src/zap.c

@@ -5999,7 +5999,7 @@ makewish(void)
            to retain wishless conduct */
         livelog_printf(LL_WISH, "declined to make a wish");
         return;
-    } else if (otmp == &cg.zeroobj) {
+    } else if (otmp == &gi.invalid_obj) {
         /* wizard mode terrain wish: skip livelogging, etc */
         return;
     }