This fixes a segfault when decoding multi-channel MP3onMP4 files.

This is similar to commit cb72230d for MPADSPContext.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 151dbe45)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>