Fix race condtion that cause corrupted tail of output TS

[moveman]

We found that occasioinally the tail of the TS output is corrupted. After some debugging and guessing, the problem should be caused by this code

m_nothingToExecute = m_writeQueue.empty();

This code is not atomic. It is equivalent to this:

bool flag = m_writeQueue.empty();
m_nothingToExecute = flag;

Other threads could jump in after the first line and before the second line.
This includes BufferedFileWriter::addWriterData(). When addWriteData() is indeed be called,
the situation will be like this:

bool flag = m_writeQueue.empty(); // (true)

  m_nothingToExecute = false;
  m_writeQueue.push(data);

m_nothingToExecute = flag; // (true)

writeQueue is inserted a data and is NOT empty; however, m_nothingToExecute is set to true by the final line.
That will cause waitForWriting() to return and then close() is called before the remaining byte is flushed to the output.

[ValeZAA]

Can you provide a sample source file and output: before and after?