avcodec/x86/mpegvideodsp: Fix signedness bug in need_emu

      Fixes: out of array read
Fixes: 3516/attachment-311488.dat

Found-by: Insu Yun, Georgia Tech.
Tested-by: wuninsu@gmail.com
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 58cf31cee7a456057f337b3102a03206d833d5e8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>