Properly handle non-media probes

libwebrtc has started sending media probes on an unannounced SSRC(0).
Currently Pion will ignore this as the SSRC hasn't been declared
explicitly and no RID/MID RTP Headers.

This adds a special case to accept SSRC 0 and Read the RTP packets. This
allows the TWCC reports to properly be generated.

interceptor_test.go

@@ -225,12 +225,59 @@ func Test_InterceptorRegistry_Build(t *testing.T) {
 		},
 	})
 
-	peerConnectionA, err := NewAPI(WithInterceptorRegistry(ir)).NewPeerConnection(Configuration{})
-	assert.NoError(t, err)
-
-	peerConnectionB, err := NewAPI(WithInterceptorRegistry(ir)).NewPeerConnection(Configuration{})
+	peerConnectionA, peerConnectionB, err := NewAPI(WithInterceptorRegistry(ir)).newPair(Configuration{})
 	assert.NoError(t, err)
 
 	assert.Equal(t, 2, registryBuildCount)
 	closePairNow(t, peerConnectionA, peerConnectionB)
 }
+
+func Test_Interceptor_ZeroSSRC(t *testing.T) {
+	to := test.TimeOut(time.Second * 20)
+	defer to.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	track, err := NewTrackLocalStaticRTP(RTPCodecCapability{MimeType: MimeTypeVP8}, "video", "pion")
+	assert.NoError(t, err)
+
+	offerer, answerer, err := newPair()
+	assert.NoError(t, err)
+
+	_, err = offerer.AddTrack(track)
+	assert.NoError(t, err)
+
+	probeReceiverCreated := make(chan struct{})
+
+	go func() {
+		sequenceNumber := uint16(0)
+		for range time.NewTicker(time.Millisecond * 20).C {
+			track.mu.Lock()
+			if len(track.bindings) == 1 {
+				_, err = track.bindings[0].writeStream.WriteRTP(&rtp.Header{
+					Version:        2,
+					SSRC:           0,
+					SequenceNumber: sequenceNumber,
+				}, []byte{0, 1, 2, 3, 4, 5})
+				assert.NoError(t, err)
+			}
+			sequenceNumber++
+			track.mu.Unlock()
+
+			if nonMediaBandwidthProbe, ok := answerer.nonMediaBandwidthProbe.Load().(*RTPReceiver); ok {
+				assert.Equal(t, len(nonMediaBandwidthProbe.Tracks()), 1)
+				close(probeReceiverCreated)
+				return
+			}
+		}
+	}()
+
+	assert.NoError(t, signalPair(offerer, answerer))
+
+	peerConnectionConnected := untilConnectionState(PeerConnectionStateConnected, offerer, answerer)
+	peerConnectionConnected.Wait()
+
+	<-probeReceiverCreated
+	closePairNow(t, offerer, answerer)
+}