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>
(cherry picked from commit d270c3202539e8364c46410e15f7570800e33343)

Conflicts:

	libavcodec/utils.c