Small improvement in sub-pixel motion search

If the current best mv(namely, the search center) is still the best mv
after the first level search, the second level checks is skipped. This
patch doesn't change the bitstream. At speed 0, it speeds up the encoder
by 1% - 2%.

Change-Id: I054c91b884d3f7aef157436c061744562bd6506d

vp9/encoder/vp9_mcomp.c

@@ -735,7 +735,7 @@ int vp9_find_best_sub_pixel_tree(const MACROBLOCK *x,
       bc = tc;
     }
 
-    if (iters_per_step > 1)
+    if (iters_per_step > 1 && best_idx != -1)
       SECOND_LEVEL_CHECKS;
 
     tr = br;