Null pointer protection in AudioPreview.java for Music app

Added protection from null pointer exception of PreviewPlayer.
It can happen if user presses play-pause or drags SeekBar
and exits from AudioPreview simultaneously, so pointer to
PreviewPlayer will be used after nullification but before
the exiting from AudioPreview activity will be finished.

Change-Id: Idb3a903fdaa95727ad63dda8cab4589b5cfacd6a
1 file changed