avcodec/alsdec: skip cases where the master channel equals the current channel

Fixes: msan_uninit-mem_7f6c73c97cf9_3571_als_04_2ch48k16b.mp4
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Reviewed-by: Thilo Borgmann <thilo.borgmann@mail.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
