Concurrent calls to the Wenxin model, and the exception problem when obtaining the token is fixed

[wochuideng]

第一个请求获取到BaiduAccessToken对象就释放了锁，此时对象中的access_token还是空。第二个请求马上进来直接拿走这个对象，导致获取到的access_token为空，进而导致请求模型报错

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