fix a bug when null function pointer is used.

For certain partition size, the function poniter may not be intialized
at all. The patch prevent the call if the pointer is not set.

Change-Id: I78b8c3992b639e8799a16b3c74f0973d07b8b9ac

vp9/encoder/vp9_mcomp.c

@@ -1824,7 +1824,7 @@ int vp9_full_search_sadx8(MACROBLOCK *x, int_mv *ref_mv,
       }
     }
 
-    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);