opusdec: make sure all substreams have the same number of coded samples

Fixes invalid writes with invalid multichannel streams.

CC:libav-stable@libav.org
(cherry picked from commit 1973079417e8701b52ba810a72cb6c7c6f7f9a56)
Signed-off-by: Anton Khirnov <anton@khirnov.net>