Fix cvar, ivar, const and method can be removed to frozen object

src/variable.c

@@ -521,6 +521,7 @@ mrb_obj_iv_inspect(mrb_state *mrb, struct RObject *obj)
 MRB_API mrb_value
 mrb_iv_remove(mrb_state *mrb, mrb_value obj, mrb_sym sym)
 {
+  mrb_check_frozen(mrb, mrb_obj_ptr(obj));
   if (obj_iv_p(obj)) {
     iv_tbl *t = mrb_obj_ptr(obj)->iv;
     mrb_value val;