Cherry-pick potential divide-by-zero fix

source/libvpx/vp9/common/vp9_prob.h

Index: source/libvpx/vp9/common/vp9_prob.h
diff --git a/source/libvpx/vp9/common/vp9_prob.h b/source/libvpx/vp9/common/vp9_prob.h
index bc1511a5e1875c484bbbccb984849bf480a6e25f..c69c62c81f85517ab3cfb4897b99d468190cafe0 100644
--- a/source/libvpx/vp9/common/vp9_prob.h
+++ b/source/libvpx/vp9/common/vp9_prob.h
@@ -33,6 +33,8 @@ typedef int8_t vp9_tree_index;
 
 #define vp9_complement(x) (255 - x)
 
+#define MODE_MV_COUNT_SAT 20
+
 /* We build coding trees compactly in arrays.
    Each node of the tree is a pair of vp9_tree_indices.
    Array index often references a corresponding probability table.
@@ -69,9 +71,28 @@ static INLINE vp9_prob merge_probs(vp9_prob pre_prob,
   return weighted_prob(pre_prob, prob, factor);
 }
 
+// MODE_MV_MAX_UPDATE_FACTOR (128) * count / MODE_MV_COUNT_SAT;
+static const int count_to_update_factor[MODE_MV_COUNT_SAT + 1] = {
+  0, 6, 12, 19, 25, 32, 38, 44, 51, 57, 64,
+  70, 76, 83, 89, 96, 102, 108, 115, 121, 128
+};
+
+static INLINE vp9_prob mode_mv_merge_probs(vp9_prob pre_prob,
+                                           const unsigned int ct[2]) {
+  const unsigned int den = ct[0] + ct[1];
+  if (den == 0) {
+    return pre_prob;
+  } else {
+    const unsigned int count = MIN(den, MODE_MV_COUNT_SAT);
+    const unsigned int factor = count_to_update_factor[count];
+    const vp9_prob prob =
+        clip_prob(((int64_t)(ct[0]) * 256 + (den >> 1)) / den);
+    return weighted_prob(pre_prob, prob, factor);
+  }
+}
+
 void vp9_tree_merge_probs(const vp9_tree_index *tree, const vp9_prob *pre_probs,
-                          const unsigned int *counts, unsigned int count_sat,
-                          unsigned int max_update_factor, vp9_prob *probs);
+                          const unsigned int *counts, vp9_prob *probs);
 
 
 DECLARE_ALIGNED(16, extern const uint8_t, vp9_norm[256]);