libav: fix problem decoding broken rv40 sample

The sample begins with a bframe which causes the rv40 to throw an error
at the beginning of decode.  This error leaves frame threading in a bad
state that is never recovered from.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6296 b64f7644-9d1e-0410-96f1-a4d463321fa5