Performance: _privatemsg_get_new_messages() does database query even for anonymous users

To make it clearer what the patch does, I'll print out the change here, so people don't have to download the patch and analyse.

Basically the patch changes the _privatemsg_get_new_messages() function so that requests for private messages for uid 0 don't happen (there's no point in getting private messages for a non-logged-in user.

So the code was:

function _privatemsg_get_new_messages($uid = 0) {
  global $user;
  static $cache = array();
  if ($uid == 0) {
    $uid = $user->uid;
  }
  if (!isset($cache[$uid])) {
    $cache[$uid] = (int)db_result(db_query('SELECT COUNT(*) FROM {privatemsg} WHERE recipient = %d AND newmsg = 1 AND recipient_del = 0', $uid));
  }
  return $cache[$uid];
}

And was changed to:

function _privatemsg_get_new_messages($uid = 0) {
  global $user;
  static $cache = array();
  if ($uid == 0) {
    $uid = $user->uid;
  }
  
  /** Running the query to grab private messages for non-logged-in users is 
   * pointless and a performance drain. Only look for private messages for real
   * users.  */
  if ($uid == 0) {
  	return array();
  }
  if (!isset($cache[$uid])) {
    $cache[$uid] = (int)db_result(db_query('SELECT COUNT(*) FROM {privatemsg} WHERE recipient = %d AND newmsg = 1 AND recipient_del = 0', $uid));
  }
  return $cache[$uid];
}

Perhaps another approach:

  /** Running the query to grab private messages for non-logged-in users is
   * pointless and a performance drain. Only look for private messages for real
   * users.  */
  if ($uid == 0) {
      $cache[0] = 0;
  }

But I think there's something less clean about that. In some ways what we should be returning is a false for uid 0, but that would break the receiving code since it expects an int.