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/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)) {