avcodec/mpeg4videodec: Clear sprite wraping on unsupported cases in VOP decode

Fixes: Integer overflow
Fixes: 1572/clusterfuzz-testcase-minimized-4578773729017856

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