henrika c103653113 Avoids WebRtcAudioTrack null pointer access at stop.
Example of new stop sequence:

PID   TID
5155  5189 I WebRtcAudioTrack: stopPlayout
5155  5189 I WebRtcAudioTrack: underrun count: 0
5155  5189 I WebRtcAudioTrack: stopThread
5155  5189 I WebRtcAudioTrack: Stopping the AudioTrackThread...

5155  5236 I WebRtcAudioTrack: Stopping and flushing the audio track...
5155  5236 I WebRtcAudioTrack: The audio track has now been stopped.

5155  5189 I WebRtcAudioTrack: AudioTrackThread has now been stopped.
5155  5189 I WebRtcAudioTrack: releaseAudioResources

BUG=b/64692432

Review-Url: https://codereview.webrtc.org/3001703002
Cr-Commit-Position: refs/heads/master@{#19370}
2017-08-16 13:14:08 +00:00
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