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
diff --git a/media/libmedia/AudioTrack.cpp b/media/libmedia/AudioTrack.cpp
index e3beba5..8e0704f 100644
--- a/media/libmedia/AudioTrack.cpp
+++ b/media/libmedia/AudioTrack.cpp
@@ -2244,6 +2244,9 @@
             return true;
         }
     }
+    if (exitPending()) {
+        return false;
+    }
     nsecs_t ns = mReceiver.processAudioBuffer();
     switch (ns) {
     case 0: