AudioRecord: fix AudioRecordThread crash upon exit

In case of error during openRecord_l(), there is a possibility
that a newly created AudioRecordThread 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.

Test: test case at bug
Bug: 29993798
Change-Id: I15d4f138218f6f01a020c4fc568a5b4831cb92a7