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

Change-Id: Ia78564258f42ed29545ec4f9997cc6ea83798ad0