libavcodec/mpeg4videodec: Convert sprite_offset to 64bit

This avoids intermediates from overflowing (the final values are checked)
Fixes: runtime error: signed integer overflow: -167712 + -2147352576 cannot be represented in type 'int'

Fixes: 1298/clusterfuzz-testcase-minimized-5955580877340672

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