AudioMixer: fix crash in destructor

Commit deeb1282 made that track->resampler is only set to NULL when
getTrackName() is called. It remains uninitialized for all unused tracks
causing a crash when deleted by Audiomixer destructor.

Issue 6309441.

Change-Id: I49451387a6afe81299b8368ef206524df32f74c4
1 file changed