Minor optimization in mv entropy adaptation

Adds a check to exit from the increment_nmv_count function when the
increment is 0.

Change-Id: I99c1e342d351f7800e23590f9c2419881bf1d708

vp9/common/vp9_entropymv.c

@@ -138,6 +138,7 @@ static void increment_nmv_component(int v,
                                     int incr,
                                     int usehp) {
   int s, z, c, o, d, e, f;
+  if (!incr) return;
   assert (v != 0);            /* should not be zero */
   s = v < 0;
   mvcomp->sign[s] += incr;