[c++ reader] resultRowBatch deletion bug

[yuly16]

The erroneous query:

SELECT SearchPhrase from hits WHERE SearchPhrase <> '' limit 10;

When processing the first pxl file, we can acquire enough result, thus we don't need to invoke readBatch for the remaining pxl files. However, we allocate memory for resultRowBatch in the function readBatch and delete the memory in the destructor of PixelsRecordReaderImpl. If readBatch is not invoked, we still delete resultRowBatch in the destructor, which is a nullptr.

[bianhq]

Done.