avcodec/mpeg4videodec: Fix integer overflow in num_sprite_warping_points=2 case

Fixes: runtime error: signed integer overflow: 131072 + 2147352576 cannot be represented in type 'int'
Fixes: 2192/clusterfuzz-testcase-minimized-5370387988742144

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