avcodec/fmvc: Fix off by 1 error

Fixes: out of array access
Fixes: 1643/clusterfuzz-testcase-minimized-6117573403869184

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
