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/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 */