Fixes: out of array access
Fixes: 23245/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IFF_ILBM_fuzzer-5723121327013888.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 51225dee0a6266780d26d43bd6802bbcf736327e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>