Fix possible crash in tcptransport when server is nil

libp2p/transports/tcptransport.nim

@@ -50,8 +50,10 @@ method close*(t: TcpTransport): Future[void] {.async, gcsafe.} =
   trace "stopping transport"
   await procCall Transport(t).close() # call base
 
-  t.server.stop()
-  t.server.close()
+  # server can be nil
+  if t.server != nil:
+    t.server.stop()
+    t.server.close()
   trace "transport stopped"
 
 method listen*(t: TcpTransport,