restore arena after mrb_define_method()

src/class.c

@@ -293,10 +293,12 @@ void
 mrb_define_method_id(mrb_state *mrb, struct RClass *c, mrb_sym mid, mrb_func_t func, int aspec)
 {
   struct RProc *p;
+  int ai = mrb_gc_arena_save(mrb);
 
   p = mrb_proc_new_cfunc(mrb, func);
   p->target_class = c;
   mrb_define_method_raw(mrb, c, mid, p);
+  mrb_gc_arena_restore(mrb, ai);
 }
 
 void