avcodec/hevcdec: do not let updated extradata corrupt state

Fixes: out of array access
Fixes: 2451/clusterfuzz-testcase-minimized-4781613957251072

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
