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.

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.");