Prevent access to the last step of the installation if Piwik is already installed

The last step is now also not optional, you need to visit the page to mark installation as finished

plugins/Installation/Controller.php

@@ -363,8 +363,6 @@ class Controller extends \Piwik\Plugin\ControllerAdmin
     {
         $this->checkPiwikIsNotInstalled();
 
-        $this->markInstallationAsCompleted();
-
         $view = new View(
             '@Installation/trackingCode',
             $this->getInstallationSteps(),
@@ -396,6 +394,8 @@ class Controller extends \Piwik\Plugin\ControllerAdmin
      */
     public function finished()
     {
+        $this->checkPiwikIsNotInstalled();
+
         $this->markInstallationAsCompleted();
 
         $view = new View(