Fixes: out of array access
Fixes: 49271/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5424984922652672

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 009ef35d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>