ffmpeg: Check for parameter changes at the output of the audio filter graph

It is currently possible that such changes leak through from the decoder as
well as a filter could generate such changes itself.
This commit blocks such changed packets unless the encoder declares support.

Fixes out of array reads

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
