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
am: 1d9ec790db

Change-Id: Ic155f434d944c08e1104e5bbeccfbf1ca9cd941d