aom_dsp/get_prob: relocate den == 0 test

to get_binary_prob(). the only other caller mode_mv_merge_probs() does
its own test on 0.

cherry-picked from libvpx:
93c823e24 vpx_dsp/get_prob: relocate den == 0 test

Change-Id: Ie0604ad405a97ed754e4b88c6d580eb4894ea0f6

aom_dsp/prob.h

@@ -12,6 +12,8 @@
 #ifndef AOM_DSP_PROB_H_
 #define AOM_DSP_PROB_H_
 
+#include <assert.h>
+
 #include "./aom_config.h"
 #include "./aom_dsp_common.h"
 
@@ -54,7 +56,7 @@ typedef int8_t aom_tree_index;
 typedef const aom_tree_index aom_tree[];
 
 static INLINE aom_prob get_prob(unsigned int num, unsigned int den) {
-  if (den == 0) return 128u;
+  assert(den != 0);
   {
     const int p = (int)(((int64_t)num * 256 + (den >> 1)) / den);
     // (p > 255) ? 255 : (p < 1) ? 1 : p;
@@ -64,7 +66,9 @@ static INLINE aom_prob get_prob(unsigned int num, unsigned int den) {
 }
 
 static INLINE aom_prob get_binary_prob(unsigned int n0, unsigned int n1) {
-  return get_prob(n0, n0 + n1);
+  const unsigned int den = n0 + n1;
+  if (den == 0) return 128u;
+  return get_prob(n0, den);
 }
 
 /* This function assumes prob1 and prob2 are already within [1,255] range. */