Fixed panics with nil stats (#517)

pkg/sfu/buffer/buffer.go

@@ -677,6 +677,9 @@ func (b *Buffer) GetStats() *StreamStatsWithLayers {
 	}
 
 	stats := b.rtpStats.ToProto()
+	if stats == nil {
+		return nil
+	}
 
 	layers := make(map[int]LayerStats)
 	layers[0] = LayerStats{

pkg/sfu/downtrack.go

@@ -1201,6 +1201,9 @@ func (d *DownTrack) GetTrackStats() map[uint32]*buffer.StreamStatsWithLayers {
 	streamStats := make(map[uint32]*buffer.StreamStatsWithLayers, 1)
 
 	stats := d.rtpStats.ToProto()
+	if stats == nil {
+		return nil
+	}
 
 	layers := make(map[int]buffer.LayerStats)
 	layers[0] = buffer.LayerStats{

pkg/sfu/receiver.go

@@ -420,6 +420,9 @@ func (w *WebRTCReceiver) GetTrackStats() map[uint32]*buffer.StreamStatsWithLayer
 		}
 
 		sswl := buff.GetStats()
+		if sswl == nil {
+			continue
+		}
 
 		// if simulcast, patch buffer stats with correct layer
 		if w.isSimulcast {