DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: 0338b14e28  -s ours am: 648b682eeb am: d558903f8c  -s ours am: ad1141b06b am: fbe82fe0fa
am: 7e8c500e2c

Change-Id: I4c90ec065dc57e313fea14977e22c6b90e1dc386