DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: 0338b14e28  -s ours am: 648b682eeb am: d558903f8c  -s ours am: ad1141b06b am: fbe82fe0fa am: 7e8c500e2c
am: 5eea83feb7  -s ours

Change-Id: Ie2269fabb21183e23e4b219fb08ec8c63c2eb277