Make `mrb_obj_instance_eval` to use `mrb_singleton_class_ptr`; #4973

src/vm.c

@@ -723,15 +723,11 @@ mrb_value
 mrb_obj_instance_eval(mrb_state *mrb, mrb_value self)
 {
   mrb_value a, b;
-  mrb_value cv;
-  struct RClass *c;
 
   if (mrb_get_args(mrb, "|S&", &a, &b) == 1) {
     mrb_raise(mrb, E_NOTIMP_ERROR, "instance_eval with string not implemented");
   }
-  cv = mrb_singleton_class(mrb, self);
-  c = mrb_class_ptr(cv);
-  return eval_under(mrb, self, b, c);
+  return eval_under(mrb, self, b, mrb_singleton_class_ptr(mrb, self));
 }
 
 MRB_API mrb_value