swscale/x86/rgb2rgb_template: fix crash with tiny size and nv12 output

Fixes Ticket4151
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/x86/rgb2rgb_template.c

@@ -1887,6 +1887,7 @@ static void RENAME(interleaveBytes)(const uint8_t *src1, const uint8_t *src2, ui
     for (h=0; h < height; h++) {
         int w;
 
+        if (width >= 16)
 #if COMPILE_TEMPLATE_SSE2
         __asm__(
             "xor              %%"REG_a", %%"REG_a"  \n\t"