Fixes out of array access
Fixes: ffmpeg_opus_crash1.ogg

This solution is likely not optimal in terms of error concealment but
its simple and fixes the out of array access.
Found-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Tested-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>