avcodec/zmbv: Check that the buffer is large enough for mvec

Fixes: Timeout
Fixes: 4143/clusterfuzz-testcase-4736864637419520

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