update sql-query to new format

Author: Florian Schaal

server/lib/classes/cron.d/600-purge_mailboxes.inc.php

@@ -53,8 +53,8 @@ public function onBeforeRun() {
 	public function onRunJob() {
 		global $app, $conf;
 
-		$sql = "SELECT email FROM mail_user WHERE maildir_format = 'mdbox' AND server_id = ".$server_id;
-		$records = $app->db->queryAllRecords($sql);
+		$sql = "SELECT email FROM mail_user WHERE maildir_format = 'mdbox' AND server_id = ?";
+		$records = $app->db->queryAllRecords($sql, $server_id);
 
 		if(is_array($records)) {
 			foreach($records as $rec){