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/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;
     }