Add clamp to prevent out of bound access

For bad input data, the decoder may access the array out of bounds. The
commit added clamp to prevent such out of bound access

Change-Id: I0a1cfd9b8786ea7113a998053c76605c963b077a

vp9/decoder/vp9_dsubexp.c

@@ -48,8 +48,6 @@ static int merge_index(int v, int n, int modulus) {
 
 static int inv_remap_prob(int v, int m) {
   static int inv_map_table[MAX_PROB - 1] = {
-    // generated by:
-    //   inv_map_table[j] = merge_index(j, MAX_PROB - 1, MODULUS_PARAM);
       6,  19,  32,  45,  58,  71,  84,  97, 110, 123, 136, 149, 162, 175, 188,
     201, 214, 227, 240, 253,   0,   1,   2,   3,   4,   5,   7,   8,   9,  10,
      11,  12,  13,  14,  15,  16,  17,  18,  20,  21,  22,  23,  24,  25,  26,
@@ -66,9 +64,11 @@ static int inv_remap_prob(int v, int m) {
     190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 202, 203, 204, 205,
     206, 207, 208, 209, 210, 211, 212, 213, 215, 216, 217, 218, 219, 220, 221,
     222, 223, 224, 225, 226, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237,
-    238, 239, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252,
+    238, 239, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252
   };
-  // v = merge_index(v, MAX_PROBS - 1, MODULUS_PARAM);
+  // The clamp is not necessary for conforming VP9 stream, it is added to
+  // prevent out of bound access for bad input data
+  v = clamp(v, 0, 253);
   v = inv_map_table[v];
   m--;
   if ((m << 1) <= MAX_PROB) {