cdgraphics: switch to bytestream2

Fixes possible invalid memory accesses on corrupted data.

CC:libav-stable@libav.org
Bug-ID: CVE-2013-3674
(cherry picked from commit a1599f3f7ea8478d1f6a95e59e3bc6bc86d5f812)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
cdgraphics: do not return 0 from the decode function

0 means no data consumed, so it can trigger an infinite loop in the
caller.

CC:libav-stable@libav.org
(cherry picked from commit c7d9b473e28238d4a4ef1b7e8b42c1cca256da36)
Signed-off-by: Anton Khirnov <anton@khirnov.net>