Improve checkNegotiationNeeded replaceTrack

replaceTrack with nil would cause a crash

peerconnection.go

@@ -400,6 +400,14 @@ func (pc *PeerConnection) checkNegotiationNeeded() bool { //nolint:gocognit
 					return true
 				}
 				track := sender.Track()
+				if track == nil {
+					// Situation when sender's track is nil could happen when
+					// a) replaceTrack(nil) is called
+					// b) removeTrack() is called, changing the transceiver's direction to inactive
+					// As t.Direction() in this branch is either sendrecv or sendonly, we believe (a) option is the case
+					// As calling replaceTrack does not require renegotiation, we skip check for this transceiver
+					continue
+				}
 				if !okMsid || descMsid != track.StreamID()+" "+track.ID() {
 					return true
 				}

peerconnection_renegotiation_test.go

@@ -1323,3 +1323,29 @@ func TestNegotiationNeededWithRecvonlyTrack(t *testing.T) {
 
 	closePairNow(t, pcOffer, pcAnswer)
 }
+
+func TestNegotiationNotNeededAfterReplaceTrackNil(t *testing.T) {
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	pcOffer, err := NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+
+	pcAnswer, err := NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+
+	tr, err := pcOffer.AddTransceiverFromKind(RTPCodecTypeAudio)
+	assert.NoError(t, err)
+
+	assert.NoError(t, signalPair(pcOffer, pcAnswer))
+
+	assert.NoError(t, tr.Sender().ReplaceTrack(nil))
+
+	assert.False(t, pcOffer.checkNegotiationNeeded())
+
+	assert.NoError(t, pcOffer.Close())
+	assert.NoError(t, pcAnswer.Close())
+}