avcodec/dvdsub_parser: Fix length check for short packets

The DVD subtitle parser handles two types of packets: "normal"
packets with a 16-bit length, and HD-DVD packets that set the
16-bit length to 0 and encode a 32-bit length in the next four
bytes. This implies that HD-DVD packets are at least six bytes
long, but the code didn't actually verify this.

The faulty length check results in an out of bounds read for
zero-length "normal" packets that occur in the input, which are
only 2 bytes long, but get misinterpreted as an HD-DVD packet.
When this happens the parser reads packet_len from beyond the
end of the input buffer. The subtitle stream is not correctly
decoded after this point due to the garbage packet_len.

Fixing this is pretty simple: fix the length check so packets
less than 6 bytes long will not be mistakenly parsed as HD-DVD
packets.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/dvdsub_parser.c

@@ -43,7 +43,7 @@ static int dvdsub_parse(AVCodecParserContext *s,
     *poutbuf_size = buf_size;
 
     if (pc->packet_index == 0) {
-        if (buf_size < 2 || AV_RB16(buf) && buf_size < 6) {
+        if (buf_size < 2 || (AV_RB16(buf) == 0 && buf_size < 6)) {
             if (buf_size)
                 av_log(avctx, AV_LOG_DEBUG, "Parser input %d too small\n", buf_size);
             return buf_size;