dsp: fix diff_bytes_mmx() with small width

Fixes Ticket1068
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/dsputilenc_mmx.c

@@ -823,6 +823,7 @@ static int vsad16_mmx2(void *v, uint8_t * pix1, uint8_t * pix2, int line_size, i
 
 static void diff_bytes_mmx(uint8_t *dst, const uint8_t *src1, const uint8_t *src2, int w){
     x86_reg i=0;
+    if(w>=16)
     __asm__ volatile(
         "1:                             \n\t"
         "movq  (%2, %0), %%mm0          \n\t"