DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: 25d34cacde am: 428493c16d  -s ours am: c2af35e25d am: 4da47f34d9
am: 72d1936749

Change-Id: Ia8e66d32a78e2047aebc549c0b19c373b5b25ea1