Created by: JiayiFeng

There are two bugs in our current code:

1. In the double buffer reader: The double buffer reader creates a prefetching thread to read data from files and yield them into a member variable bufer_. However, if the double buffer itself is destroyed before the prefetching thread, it will raise a segment fault because the thread is still trying to yield data to a destroyed address.

2. In the shuffle reader: https://github.com/PaddlePaddle/Paddle/blob/d7d0c1eb0be54611ad093dc210d48317a678a9bb/paddle/fluid/operators/reader/create_shuffle_reader_op.cc#L49-L65 The ReadIntoBuffers () is invoked by ShuffleReader's constructor. So if the underlying reader holds an empty file, an exception will be thrown by line 53. That is unexpected. The exception of 'no next data' should be thrown in ReadNext(), not here.