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

Change-Id: I36113550446744b6a96f9e080245d2e549dfb177