avformat/rtpdec_mpeg4: reject zero-length AU header sections

Reject AU header sections with a signaled length of zero in
rtp_parse_mp4_au().

The AU-headers-length field specifies the length in bits of the AU header
section that immediately follows. A zero-length section is not useful input
for this parser and can lead to invalid downstream state, so reject it
up front together with oversized values.

*Vulnerability reported by Zhenpeng (Leo) Lin at depthfirst*
*Patch validated by Zheng Yu at depthfirst*

Fixes: OOB read

libavformat/rtpdec_mpeg4.c

@@ -138,7 +138,7 @@ static int rtp_parse_mp4_au(PayloadContext *data, const uint8_t *buf, int len)
        length in bits */
     au_headers_length = AV_RB16(buf);
 
-    if (au_headers_length > RTP_MAX_PACKET_LENGTH)
+    if (au_headers_length == 0 || au_headers_length > RTP_MAX_PACKET_LENGTH)
       return -1;
 
     data->au_headers_length_bytes = (au_headers_length + 7) / 8;