extractor: More NULL checks...

This is blowing up if media unrecognized by ffmpeg goes through

Change-Id: Idfb2dcf581e3195f05772d375d315a6aedeab993
extractor: Underp mpeg4 fastpath

Only run this _after_ validating the result of avformat_open_input()

Change-Id: Ib7969a7f723ba58219476ceb541e8dd24c7c922b