avcodec/xan: Check for bitstream end in xan_huffman_decode()

Fixes: Timeout
Fixes: 3707/clusterfuzz-testcase-6465922706440192

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4b51437dcc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/xan.c

@@ -131,7 +131,10 @@ static int xan_huffman_decode(uint8_t *dest, int dest_len,
         return ret;
 
     while (val != 0x16) {
-        unsigned idx = val - 0x17 + get_bits1(&gb) * byte;
+        unsigned idx;
+        if (get_bits_left(&gb) < 1)
+            return AVERROR_INVALIDDATA;
+        idx = val - 0x17 + get_bits1(&gb) * byte;
         if (idx >= 2 * byte)
             return AVERROR_INVALIDDATA;
         val = src[idx];