DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: 0338b14e28  -s ours am: 648b682eeb am: d558903f8c  -s ours am: ad1141b06b am: fbe82fe0fa am: 7e8c500e2c am: 5eea83feb7  -s ours am: 6e39a8cb87
am: 701bc80a92

Change-Id: Ie621e62e55db3c23d5fbd2bfc54efddc9696e5cc