fix missing check allocator is reference in ClearStack()

[xiegx94]

ClearStack() function in document.h misses check allocator type. This would cause memory leak if parse failed. Such as following code:

int main()
{
    if (1) {
      rapidjson::Document doc;
      auto& ret = doc.Parse<0>(R"(
        {"a":"a","b":"b"}, {"c":"c"}
      )");
    }
    return 0;
}

[tencent-adm]

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