Close connection if frpc can't connection to local server

Now, when frpc can't connect to local server it leaves the connection alone, the patch fix it.

Fixed #575

client/proxy.go

@@ -434,6 +434,7 @@ func HandleTcpWorkConnection(localInfo *config.LocalSvrConf, proxyPlugin plugin.
 		localConn, err := frpNet.ConnectServer("tcp", fmt.Sprintf("%s:%d", localInfo.LocalIp, localInfo.LocalPort))
 		if err != nil {
 			workConn.Error("connect to local service [%s:%d] error: %v", localInfo.LocalIp, localInfo.LocalPort, err)
+			remote.Close()
 			return
 		}