AudioTrack: fix AudioTrackThread crash upon exit

In case of error during createTrack_l(), there is a possibility
that a newly created AudioTrackThread is resumed by requestExit()
just after entering the Thread class _threadLoop() but before
entering the class threadLoop(). In this case, processAudioBuffer()
is executed once with mCblk == 0 and we assert.

Bug: 17894033.
Change-Id: I93d0d0d4dbf14bcb88dad23e6ad49d2ecd47badd