webp: validate the distance prefix code

According to the WebP Lossless Bitstream Specification the highest
allowed value for a prefix code is 39.

If prefix_code is too large, the calculated extra_bits has an invalid
value and triggers an assertion in get_bits.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/webp.c

@@ -688,6 +688,11 @@ static int decode_entropy_coded_image(WebPContext *s, enum ImageRole role,
                 length = offset + get_bits(&s->gb, extra_bits) + 1;
             }
             prefix_code = huff_reader_get_symbol(&hg[HUFF_IDX_DIST], &s->gb);
+            if (prefix_code > 39) {
+                av_log(s->avctx, AV_LOG_ERROR,
+                       "distance prefix code too large: %d\n", prefix_code);
+                return AVERROR_INVALIDDATA;
+            }
             if (prefix_code < 4) {
                 distance = prefix_code + 1;
             } else {