Fix possible crash in tcptransport when server is nil

libp2p/transports/tcptransport.nim

@@ -50,6 +50,8 @@ method close*(t: TcpTransport): Future[void] {.async, gcsafe.} =
   trace "stopping transport"
   await procCall Transport(t).close() # call base
 
+  # server can be nil
+  if t.server != nil:
     t.server.stop()
     t.server.close()
   trace "transport stopped"