frameworks/av : Add Null check for Audio Track

- Check if AudioTrack is available before de-referencing
it's pointer to avoid crashes during stability runs

- Rootcause: During Video playback when the audioTrack
init fails due to mediaplayer errors, the getMediaTimeUs
is already getting called resulting in a crash

- Fix: Added NULL pointer check when de-referencing pointer

CRs-Fixed: 658536
Change-Id: I0846e5cd805cb1523edb52ea9c7c04b41f954e6f