Set a default mv entropy if no tokens are coded.

This doesn't affect the result, since there are no MVs coded using this
entropy. It does, however, silence valgrind warnings about uninitialized
variables.

Change-Id: I6e21ba92df6ce5381bf58b8c349ef4373294a0b6

vp8/common/entropymv.c

@@ -117,6 +117,8 @@ __inline static void calc_prob(vp8_prob *p, const unsigned int ct[2], int pbits)
   if (tot) {
     const vp8_prob x = ((ct[0] * 255) / tot) & -(1 << (8 - pbits));
     *p = x ? x : 1;
+  } else {
+    *p = 128;
   }
 }