Check if oc_token cookie exists before trying to use it

lib/base.php

@@ -557,7 +557,9 @@ class OC
             OC_App::loadApps();
             OC_User::setupBackends();
             if (isset($_GET["logout"]) and ($_GET["logout"])) {
+		if (isset($_COOKIE['oc_token'])) {
 			OC_Preferences::deleteKey(OC_User::getUser(), 'login_token', $_COOKIE['oc_token']);
+		}
                 OC_User::logout();
                 header("Location: " . OC::$WEBROOT . '/');
             } else {