Created by: JiayiFeng

1. Issue 1: https://github.com/PaddlePaddle/Paddle/blob/604a473acf39e62d107549fe5d2b505754ba2ffc/paddle/fluid/recordio/header.cc#L30-L37

readsome can only read a file from the buffer. But here is the first time we read from is, so it is possible that the file is not in the buffer and we can read nothing.

2. Issue 2: https://github.com/PaddlePaddle/Paddle/blob/30b70323b4dc04ff1270c520711fa5428f509ae5/paddle/fluid/recordio/scanner.cc#L30-L33

Before seekg(), we shall invoke stream_->clear() to clear steam's flags.