vp9/update_prob: prevent out of bounds table read

the max value of the lookup in expanded form is:
(((1 << 7) - 1) << 1) - 65 + 1 + 64 = 254

add one entry of padding to inv_map_table[] to prevent out of bounds
access with non-conforming / fuzzed bitstreams

Signed-off-by: James Zern <jzern@google.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vp9.c

@@ -425,7 +425,7 @@ static av_always_inline int inv_recenter_nonneg(int v, int m)
 // differential forward probability updates
 static int update_prob(VP56RangeCoder *c, int p)
 {
-    static const int inv_map_table[254] = {
+    static const int inv_map_table[255] = {
           7,  20,  33,  46,  59,  72,  85,  98, 111, 124, 137, 150, 163, 176,
         189, 202, 215, 228, 241, 254,   1,   2,   3,   4,   5,   6,   8,   9,
          10,  11,  12,  13,  14,  15,  16,  17,  18,  19,  21,  22,  23,  24,
@@ -444,7 +444,7 @@ static int update_prob(VP56RangeCoder *c, int p)
         207, 208, 209, 210, 211, 212, 213, 214, 216, 217, 218, 219, 220, 221,
         222, 223, 224, 225, 226, 227, 229, 230, 231, 232, 233, 234, 235, 236,
         237, 238, 239, 240, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251,
-        252, 253,
+        252, 253, 253,
     };
     int d;
 
@@ -474,6 +474,7 @@ static int update_prob(VP56RangeCoder *c, int p)
         if (d >= 65)
             d = (d << 1) - 65 + vp8_rac_get(c);
         d += 64;
+        av_assert2(d < FF_ARRAY_ELEMS(inv_map_table));
     }
 
     return p <= 128 ? 1 + inv_recenter_nonneg(inv_map_table[d], p - 1) :