libhb: fix problem with avcodec audio sample_rate
libavcodec does not yet consistently set AVFrame.sample_rate.
So when it is not set, use AVCodecContext.sample_rate and log a warning.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6011 b64f7644-9d1e-0410-96f1-a4d463321fa5