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

Change-Id: Ib4c116449fa9a43f7a4c1b46c793c5de2d0cbac3