correct check for duplicate email

Author: lindyblackburn

auth.php

@@ -395,7 +395,7 @@ public function modifyUser($user, $changes) {
 
  if(isset($changes['mail'])) {
  $found = $this->getUserByEmail($changes['mail']);
- if($found != $user) {
+ if($found && $found != $user) {
  msg($this->getLang('emailduplicate'), -1);
  return false;
  }