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>

libavcodec/x86/mpegvideodsp.c

@@ -52,8 +52,9 @@ static void gmc_mmx(uint8_t *dst, uint8_t *src,
     const int dyh = (dyy - (1 << (16 + shift))) * (h - 1);
     const int dxh = dxy * (h - 1);
     const int dyw = dyx * (w - 1);
-    int need_emu  =  (unsigned) ix >= width  - w ||
-                     (unsigned) iy >= height - h;
+    int need_emu  =  (unsigned) ix >= width  - w || width < w ||
+                     (unsigned) iy >= height - h || height< h
+                     ;
 
     if ( // non-constant fullpel offset (3% of blocks)
         ((ox ^ (ox + dxw)) | (ox ^ (ox + dxh)) | (ox ^ (ox + dxw + dxh)) |