DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: 25d34cacde am: 428493c16d  -s ours am: c2af35e25d am: 4da47f34d9 am: 72d1936749 am: 203ce3a3e5  -s ours am: 26ffaaef5b am: f6b9dcc895
am: ead6b68789

Change-Id: I4ff5826ae55d3df91ea27e5ea5922f160518255c