DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: de84a76b86 am: 2c76c56d56 am: f1e84ca858  -s ours am: 7b9756261d  -s ours am: d676f1bfeb am: 0ce582e5c4  -s ours am: e843c53efa am: 22d0f74490
am: 1eb29f6a11

Change-Id: Iabefaa42f7ddbb7393a441c29ebf9adbbee5bcbd