field_widget_field and field_widget_instance should be used

In user_reference_autocomplete_validate(), the following code is used:

  $field = $form_state['field'][$element['#field_name']][$element['#language']]['field'];
  $instance = $form_state['field'][$element['#field_name']][$element['#language']]['instance'];

These can be replaced with calls to field_widget_field() and field_widget_instance(). I haven't checked if there are other possible improvements in other places.