... in userprotect_form_alter... 
   case 'user_multiple_delete_confirm':
      $form['#validate'] = userprotect_add_validation($form['#validate'], array('userprotect_user_admin_account_validate' => array($form)));
      break;

...

function userprotect_add_validation($element, $validation) {

  // If it's already an array, prepend our custom validation function.
  if (is_array($element)) {
    $form = $validation + $element;
  }
  // Otherwise create it from scratch.
  else {
    $form = $validation;
  }

  return $form;
}