Fixes: out of array access
Fixes: 27434/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RSCC_fuzzer-5196757675540480
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
(cherry picked from commit a5ed6da9bd)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
tags/n4.3.2