Uninitialized pointer can cause crash.

mpAudioTrack should be initialized in any case.
If getOutputSamplingRate returns ERROR, this won't be initialized.
As a result, deleting this field in dtor can cause crash.

Change-Id: I950d353361b19635302339dac35752a2514d2d80
Signed-off-by: You Kim <you.kim72@gmail.com>