DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing. am: de84a76b86 am: 2c76c56d56 am: f1e84ca858  -s ours am: 7b9756261d  -s ours am: d676f1bfeb am: 0ce582e5c4  -s ours am: e843c53efa am: 22d0f74490 am: 1eb29f6a11 am: c8ce691ddb  -s ours am: dc1ee86987
am: 7298c1c037

Change-Id: I58a1a674370fe398aa3d05dd9e48d4a3408b77f1