Fixes: out of array write
Fixes: 63520/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLIC_fuzzer-4876198087622656
Regression since: c7f8d42c (was not posted to ffmpeg-devel)

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Sean McGovern <gseanmcg@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>