Fix setting domain in virtualmin password driver (#1489332)

plugins/password/drivers/virtualmin.php

@@ -19,7 +19,6 @@ class rcube_virtualmin_password
     function save($currpass, $newpass)
     {
         $rcmail   = rcmail::get_instance();
-
         $format   = $rcmail->config->get('password_virtualmin_format', 0);
         $username = $_SESSION['username'];
 
@@ -52,6 +51,10 @@ class rcube_virtualmin_password
             $domain = substr(strrchr($username, "@"), 1);
         }
 
+        if (!$domain) {
+            $domain = $rcmail->user->get_username('domain');
+        }
+
         $username = escapeshellcmd($username);
         $domain   = escapeshellcmd($domain);
         $newpass  = escapeshellcmd($newpass);