Fix negotiation needed panic on nil Sendonly track

In step 5.3.1, since we do not have a sender we can only choose
to signal that we need negotiation.

peerconnection.go

@@ -392,7 +392,11 @@ func (pc *PeerConnection) checkNegotiationNeeded() bool { //nolint:gocognit
 			// Step 5.3.1
 			if t.Direction() == RTPTransceiverDirectionSendrecv || t.Direction() == RTPTransceiverDirectionSendonly {
 				descMsid, okMsid := m.Attribute(sdp.AttrKeyMsid)
-				track := t.Sender().Track()
+				sender := t.Sender()
+				if sender == nil {
+					return true
+				}
+				track := sender.Track()
 				if !okMsid || descMsid != track.StreamID()+" "+track.ID() {
 					return true
 				}

peerconnection_test.go

@@ -1,6 +1,7 @@
 package webrtc
 
 import (
+	"context"
 	"reflect"
 	"sync"
 	"sync/atomic"
@@ -751,3 +752,38 @@ func TestTransportChain(t *testing.T) {
 
 	closePairNow(t, offer, answer)
 }
+
+func TestNegotiationNeededWithRecvonlyTrack(t *testing.T) {
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	pcOffer, pcAnswer, err := newPair()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	pcAnswer.OnNegotiationNeeded(wg.Done)
+
+	_, err = pcOffer.AddTransceiverFromKind(RTPCodecTypeVideo, RTPTransceiverInit{Direction: RTPTransceiverDirectionRecvonly})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := signalPair(pcOffer, pcAnswer); err != nil {
+		t.Fatal(err)
+	}
+
+	onDataChannel, onDataChannelCancel := context.WithCancel(context.Background())
+	pcAnswer.OnDataChannel(func(d *DataChannel) {
+		onDataChannelCancel()
+	})
+	<-onDataChannel.Done()
+	wg.Wait()
+
+	closePairNow(t, pcOffer, pcAnswer)
+}