fix encoding of RTP packets with padding

this fixes a SIGSEGV with GStreamer

serverudpl.go

@@ -203,6 +203,10 @@ func (u *serverUDPListener) processRTP(clientData *clientData, payload []byte) {
 		return
 	}
 
+	// remove padding
+	pkt.Header.Padding = false
+	pkt.PaddingSize = 0
+
 	now := time.Now()
 	atomic.StoreInt64(clientData.ss.udpLastFrameTime, now.Unix())
 	clientData.ss.announcedTracks[clientData.trackID].rtcpReceiver.ProcessPacketRTP(now, pkt)