ac3dec: ensure get_buffer() gets a buffer for the correct number of channels

If there is an error during frame parsing, but AVCodecContext.channels was
changed and AC3DecodeContext.out_channels was set previously, the two may not
match.

Fixes CVE-2012-2802
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

CC: libav-stable@libav.org
