avcodec/mpeg4videodec: Use 64 bit intermediates for sprite delta

Fixes: runtime error: signed integer overflow: -104713 * 65536 cannot be represented in type 'int'
Fixes: 3453/clusterfuzz-testcase-minimized-5555554657239040
Fixes: 3528/clusterfuzz-testcase-minimized-6283628420005888

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