xan: Check for out of bound reads in xan_huffman_decode()

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
(cherry picked from commit 3db3fdf4c6)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/xan.c

@@ -112,7 +112,10 @@ static int xan_huffman_decode(unsigned char *dest, int dest_len,
     init_get_bits(&gb, ptr, ptr_len * 8);
 
     while ( val != 0x16 ) {
-        val = src[val - 0x17 + get_bits1(&gb) * byte];
+        unsigned idx = val - 0x17 + get_bits1(&gb) * byte;
+        if (idx >= 2 * byte)
+            return -1;
+        val = src[idx];
 
         if ( val < 0x16 ) {
             if (dest >= dest_end)