avcodec/x86/mpegvideoenc_template: fix integer overflow

Signed-off-by: NMichael Niedermayer <michaelni@gmx.at>

libavcodec/x86/mpegvideoenc_template.c

@@ -216,7 +216,7 @@ static int RENAME(dct_quantize)(MpegEncContext *s,
         "psubusw "MM"1, "MM"4               \n\t"
         "packuswb "MM"4, "MM"4              \n\t"
 #if COMPILE_TEMPLATE_SSE2
-        "packuswb "MM"4, "MM"4              \n\t"
+        "packsswb "MM"4, "MM"4              \n\t"
 #endif
         "movd "MM"4, %0                     \n\t" // *overflow
         : "=g" (*overflow)