avcodec/mpeg4videodec: Check the other 3 sprite points for intermediate overflows

This is not necessarily specific to fuzzed files

Fixes: Multiple integer overflows
Fixes: 656/clusterfuzz-testcase-6463814516080640
Fixes: 658/clusterfuzz-testcase-6691260146384896

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