Merge "DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing." into lmp-dev am: b6ba3d7e47  -s ours am: 37e3ea354a am: ea136eb981  -s ours am: 2456e12801 am: fe97b39795 am: 3c76391550 am: 1099791bd6  -s ours am: 0e4feca95e
am: 21269e19f5

Change-Id: I9c89225a70cb21bfe032e2387e5dce2d132a82f1