peer: fix crash in the rtcp handling (#67)

* peer: fix crash in the rtcp handling

It turns out that a `receiver` may actually have a `nil` track inside.
Not sure when and why but it looks like it happens during the
negotiation (maybe when the transceiver exists, but the track does not
exist already?).

That's the body of the function:
```go
func (r *RTPReceiver) Track() *TrackRemote {
	r.mu.RLock()
	defer r.mu.RUnlock()

	if len(r.tracks) != 1 { // so this is possible!
		return nil
	}
	return r.tracks[0].track
}
```

* peer: more elegant handling of the nil in the rtcp

Co-authored-by: Šimon Brandner <[email protected]>

Co-authored-by: Šimon Brandner <[email protected]>

Author: daniel-abramov

pkg/peer/peer.go

@@ -135,7 +135,7 @@ func (p *Peer[ID]) WriteRTCP(trackID string, packets []RTCPPacketType) error {
 	// Find the right track.
 	receivers := p.peerConnection.GetReceivers()
 	receiverIndex := slices.IndexFunc(receivers, func(receiver *webrtc.RTPReceiver) bool {
-		return receiver.Track().ID() == trackID
+		return receiver.Track() != nil && receiver.Track().ID() == trackID
 	})
 	if receiverIndex == -1 {
 		return ErrTrackNotFound