A few bugs in RecordIO

[JiayiFeng]

1. Issue 1:
   

   Paddle/paddle/fluid/recordio/header.cc

   Lines 30 to 37 in 604a473

   	bool Header::Parse(std::istream& is) {
   	uint32_t magic;
   	size_t read_size =
   	is.readsome(reinterpret_cast<char*>(&magic), sizeof(uint32_t));
   	if (read_size < sizeof(uint32_t)) {
   	return false;
   	}
   	PADDLE_ENFORCE_EQ(magic, kMagicNumber);

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:
   

   Paddle/paddle/fluid/recordio/scanner.cc

   Lines 30 to 33 in 30b7032

   	void Scanner::Reset() {
   	stream_->seekg(0, std::ios::beg);
   	ParseNextChunk();
   	}

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