Fix recursion when running an update script

lib/app.php

@@ -36,6 +36,7 @@ class OC_App{
 	static private $appInfo = array();
 	static private $appTypes = array();
 	static private $loadedApps = array();
+	static private $checkedApps = array();
 
 	/**
 	 * @brief loads all apps
@@ -530,6 +531,10 @@ class OC_App{
 	 * check if the app need updating and update when needed
 	 */
 	public static function checkUpgrade($app) {
+		if (in_array($app, self::$checkedApps)) {
+			return;
+		}
+		self::$checkedApps[] = $app;
 		$versions = self::getAppVersions();
 		$currentVersion=OC_App::getAppVersion($app);
 		if ($currentVersion) {