avoid crash when using --best on cpus with SSE3 (but not SSE4) support

Change-Id: Ie100114a01b8b4da7248603c40676792cd06b32a

vp9/encoder/vp9_mcomp.c

@@ -1726,7 +1726,7 @@ int vp9_full_search_sadx3(MACROBLOCK *x, int_mv *ref_mv,
     check_here = r * mv_stride + in_what + col_min;
     c = col_min;
 
-    while ((c + 2) < col_max) {
+    while ((c + 2) < col_max && fn_ptr->sdx3f != NULL) {
       int i;
 
       fn_ptr->sdx3f(what, what_stride, check_here, in_what_stride, sad_array);