rtpdec: Fix the minimum packet length for RTCP SR packets

We actually read 20 bytes of these packets.
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavformat/rtpdec.c

@@ -115,7 +115,7 @@ static int rtcp_parse_packet(RTPDemuxContext *s, const unsigned char *buf, int l
     while (len >= 2) {
         switch (buf[1]) {
         case RTCP_SR:
-            if (len < 16) {
+            if (len < 20) {
                 av_log(NULL, AV_LOG_ERROR, "Invalid length for RTCP SR packet\n");
                 return AVERROR_INVALIDDATA;
             }