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/decl.c

@@ -440,6 +440,8 @@ const struct instance_globals_i g_init_i = {
     UNDEFINED_PTR, /* id_map */
     /* sp_lev.c */
     FALSE, /* in_mk_themerooms */
+    DUMMY,  /* invalid_obj */
+
     TRUE, /* havestate*/
     IVMAGIC  /* i_magic to validate that structure layout has been preserved */
 };