Merge "DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing." into lmp-dev am: b6ba3d7e47  -s ours am: 37e3ea354a am: ea136eb981  -s ours
am: 2456e12801

Change-Id: Ic7b4cf9994060ed34e4ad3c13f09a5f28f635ab1