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

Change-Id: I38cfc899fbe9fce3607d2ad56662d0c82691f851