DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: de84a76b86 am: 2c76c56d56 am: f1e84ca858  -s ours
am: 7b9756261d  -s ours

Change-Id: I016c0378320af03c05a01059de8c6edeabd84130