Sjoerd found this bug while doing some interop testing. RTCP compound packets don't always start with a SR packet, they could also only contain RR packets. I guess in that case it is not useful for syncing so we can just drop it.

Patch:

--- a/gst/rtpmanager/gstrtpssrcdemux.c
+++ b/gst/rtpmanager/gstrtpssrcdemux.c
@@ -482,6 +482,9 @@ gst_rtp_ssrc_demux_rtcp_chain (GstPad * pad, GstBuffer * buf)
       gst_rtcp_packet_sr_get_sender_info (&packet, &ssrc, NULL, NULL, NULL,
           NULL);
       break;
+    case GST_RTCP_TYPE_RR:
+      gst_buffer_unref (buf);
+      return GST_FLOW_OK;
     default:
       goto invalid_rtcp;
   }