Fix deadlock on write to track channel

pkg/core/track.go

@@ -97,8 +97,9 @@ func NewSender(media *Media, codec *Codec) *Sender {
 		buf:   buf,
 	}
 	s.Input = func(packet *Packet) {
-		// writing to nil chan - OK, writing to closed chan - panic
 		s.mu.Lock()
+		if s.buf != nil {
+			// unblocked write to channel
 			select {
 			case s.buf <- packet:
 				s.Bytes += len(packet.Payload)
@@ -106,6 +107,9 @@ func NewSender(media *Media, codec *Codec) *Sender {
 			default:
 				s.Drops++
 			}
+		} else {
+			s.Drops++
+		}
 		s.mu.Unlock()
 	}
 	s.Output = func(packet *Packet) {