DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: de84a76b86 am: 2c76c56d56 am: f1e84ca858  -s ours am: 7b9756261d  -s ours
am: d676f1bfeb

Change-Id: I0f8ec51bbc0e26aae780f20668f5ac03d5a8da42