Variable editor displays message indicating a variable was deleted when none was.

Fixed by testing count($deletes):

function devel_variable_form_submit($form, &$form_state) {
  $deletes = array_filter($form_state['values']['variables']);
  array_walk($deletes, 'variable_del');
  if (count($deletes)) {
    drupal_set_message(format_plural(count($deletes), 'One variable deleted.', '@count variables deleted.'));
  }
}