vm.c (check_target_class): return Object class if target is NULL.

Since #5272 target_class kept in a Proc may be NULL. It crashes
`iij/mruby-require` gem for example; close #5725

src/vm.c

@@ -1153,8 +1153,7 @@ check_target_class(mrb_state *mrb)
 {
   struct RClass *target = mrb_vm_ci_target_class(mrb->c->ci);
   if (!target) {
-    mrb_value exc = mrb_exc_new_lit(mrb, E_TYPE_ERROR, "no target class or module");
-    mrb_exc_set(mrb, exc);
+    return mrb->object_class;
   }
   return target;
 }