ac3dec: Increment channel pointers only once per channel

If the channel mapping map multiple output channels to one
input channel, we should only increment the actual pointer once.

Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
ac3dec: Consistently use AC3_BLOCK_SIZE and sizeof

Signed-off-by: Martin Storsjö <martin@martin.st>