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/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) {