catch the case where files_trashbin/files doesn't exist

apps/files_trashbin/lib/trashbin.php

@@ -922,6 +922,9 @@ class Trashbin {
 
 		$view = new \OC\Files\View('/' . $user . '/files_trashbin');
 		$dh = $view->opendir('/files');
+		if (!$dh) {
+			return false;
+		}
 		while ($file = readdir($dh)) {
 			if ($file !== '.' and $file !== '..') {
 				return false;