mlpdec: dont leave a invalid huff_lsb in the context.

Fix assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec_decode_audio: do not trust the channel layout, use the channel count.

Fixes memory corruption

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