avcodec/cavs: Check updated MV

Fixes: runtime error: signed integer overflow: 251 + 2147483647 cannot be represented in type 'int'
Fixes: 1438/clusterfuzz-testcase-minimized-4917542646710272

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