avcodec/mpeg4videodec: Fix runtime error: signed integer overflow: 134527392 * 16 cannot be represented in type 'int'

This checks the sprite delta intermediates for overflow
Fixes: 716/clusterfuzz-testcase-4890287480504320

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