Add check if user is logged-in

Otherwise a fatal PHP error is thrown since the filesystem is not setup:
```
( ! ) Fatal error: Call to a member function getOwner() on null in /Users/lreschke/Programming/core/lib/private/files/filesystem.php on line 817
Call Stack
1	0.0004	247792	{main}( )	../index.php:0
2	0.0946	5088776	OC::handleRequest( )	../index.php:28
3	0.1423	7491280	OC\Route\Router->match( )	../base.php:748
4	0.1664	8592152	call_user_func:{/Users/lreschke/Programming/core/lib/private/route/router.php:250} ( )	../router.php:250
5	0.1664	8592216	__lambda_func( )	../router.php:250
6	0.1665	8599064	require_once( '/Users/lreschke/Programming/core/apps/files_versions/ajax/getVersions.php' )	../route.php(135) : runtime-created function:1
7	0.1801	9061096	OCA\Files_Versions\Storage::getUidAndFilename( )	../getVersions.php:7
8	0.1801	9061144	OC\Files\Filesystem::getOwner( )	../storage.php:45
```

Furthermore this adds the CSRF check as a nice-to-have addition, the requests accessing this are done via the jQuery AJAX methods and thus the header is automatically added to all requests already.