RunWithUi sets appropriate default for stderr buffer

- The WinRM communicator requires that stderr is not nil
- RunWithUi will set a default stderr buffer if none is specified in the command

provisioner/windows-restart/provisioner.go

@@ -263,7 +263,7 @@ var waitForCommunicator = func(ctx context.Context, p *Provisioner) error {
 				cmdKeyCheck.Stdout = &buf
 				cmdKeyCheck.Stdout = io.MultiWriter(cmdKeyCheck.Stdout, &buf2)
 
-				err := p.comm.Start(ctx, cmdKeyCheck)
+				err := cmdKeyCheck.RunWithUi(ctx, p.comm, p.ui)
 				if err != nil {
 					log.Printf("Communication connection err: %s", err)
 					shouldContinue = true