Need to copy argv since it may be reallocated; fix #3500

The argv for `mrb_yield_with_class()` should not be on mruby stack.
Note the result from `mrb_get_args("*")` is on the stack.

mrbgems/mruby-object-ext/src/object.c

@@ -62,10 +62,11 @@ nil_to_i(mrb_state *mrb, mrb_value obj)
 static mrb_value
 mrb_obj_instance_exec(mrb_state *mrb, mrb_value self)
 {
-  mrb_value *argv;
+  const mrb_value *argv;
   mrb_int argc;
   mrb_value blk;
   struct RClass *c;
+  mrb_value args;
 
   mrb_get_args(mrb, "*&", &argv, &argc, &blk);
 
@@ -83,7 +84,8 @@ mrb_obj_instance_exec(mrb_state *mrb, mrb_value self)
     c = mrb_class_ptr(mrb_singleton_class(mrb, self));
     break;
   }
-
+  args = mrb_ary_new_from_values(mrb, argc, argv);
+  argv = RARRAY_PTR(args);
   return mrb_yield_with_class(mrb, blk, argc, argv, self, c);
 }