remove String class initialization in error.c; ref #2429

src/error.c

@@ -91,10 +91,15 @@ static mrb_value
 exc_to_s(mrb_state *mrb, mrb_value exc)
 {
   mrb_value mesg = mrb_attr_get(mrb, exc, mrb_intern_lit(mrb, "mesg"));
+  struct RObject *p;
 
   if (!mrb_string_p(mesg)) {
     return mrb_str_new_cstr(mrb, mrb_obj_classname(mrb, exc));
   }
+  p = mrb_obj_ptr(mesg);
+  if (!p->c) {
+    p->c = mrb->string_class;
+  }
   return mesg;
 }
 
@@ -436,9 +441,6 @@ mrb_init_exception(mrb_state *mrb)
 {
   struct RClass *exception, *runtime_error, *script_error;
 
-  /* initialize mrb->string_class before creating RString object for nomem_err */
-  mrb->string_class = mrb_define_class(mrb, "String", mrb->object_class);
-
   mrb->eException_class = exception = mrb_define_class(mrb, "Exception", mrb->object_class); /* 15.2.22 */
   mrb_define_class_method(mrb, exception, "exception", mrb_instance_new,  MRB_ARGS_ANY());
   mrb_define_method(mrb, exception, "exception",       exc_exception,     MRB_ARGS_ANY());