rtpproto: Write a warning if the input data written isn't RTP packetized

Tell the user that the RTP muxer needs to be used to packetize
the data - using the RTP protocol on its own isn't enough.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpproto.c

@@ -411,6 +411,10 @@ static int rtp_write(URLContext *h, const uint8_t *buf, int size)
     if (size < 2)
         return AVERROR(EINVAL);
 
+    if (buf[0] != (RTP_VERSION << 6))
+        av_log(h, AV_LOG_WARNING, "Data doesn't look like RTP packets, "
+                                  "make sure the RTP muxer is used\n");
+
     if (s->write_to_source) {
         int fd;
         struct sockaddr_storage *source, temp_source;