load_exec should return undef on syntax errors

src/parse.y

@@ -4866,7 +4866,7 @@ load_exec(mrb_state *mrb, parser_state *p, mrbc_context *c)
       static const char msg[] = "syntax error";
       mrb->exc = (struct RObject*)mrb_object(mrb_exc_new(mrb, E_SYNTAX_ERROR, msg, sizeof(msg) - 1));
       mrb_parser_free(p);
-      return mrb_nil_value();
+      return mrb_undef_value();
     }
   }
   n = mrb_generate_code(mrb, p);