update mythtranscode to work with ac3 with newer ffmpeg

ffmpeg has moved the duration/size of the audio frame to another place, so look
there

links used to figure it out how to move forward
http://ffmpeg.org/trac/ffmpeg/ticket/1240
http://git.videolan.org/?p=ffmpeg.git;a=commitdiff;h=364c71c80e9124a2624e3bfeb8e84c5cddeda222
http://code.mythtv.org/trac/ticket/2077#comment:42

thanks to Nicolas Pöhlmann for finding the commit that broke it

Refs: #2077
(cherry picked from commit a9b906528)
add NULL check missed in [a9b90652]

found by Jim Stichnoth

Refs #2077
(cherry picked from commit f04a8e42b)