MPEG4Extractor: still more NULL dereference fixes

When processing various FourCC values within MP4 media, mLastTrack is accessed
without first ensuring that a track has been encoutered. Check for NULL and
bail out instead of crashing.

Bug: 20139950
Change-Id: Ie16687024d17348f576a0e13bd60bd4d6898de91
1 file changed