form_select_options() crashes when encountering Twig_Markup objects

form_select_options() causes infinite recursion in the installer when it's trying to generate markup for the options that are objects.

I'm not sure when in the past #type => 'select' #options were objects, but now, sometimes this function is getting an $element where the #options are all Twig_Markup objects.

Here's form_select_options() from core/includes/form.inc:

function form_select_options($element, $choices = NULL) {
  if (!isset($choices)) {
    $choices = $element['#options'];
  }
  // array_key_exists() accommodates the rare event where $element['#value'] is NULL.
  // isset() fails in this situation.
  $value_valid = isset($element['#value']) || array_key_exists('#value', $element);
  $value_is_array = $value_valid && is_array($element['#value']);
  $options = '';
  foreach ($choices as $key => $choice) {
    if (is_array($choice)) {
      $options .= '<optgroup label="' . $key . '">';
      $options .= form_select_options($element, $choice);
      $options .= '</optgroup>';
    }
    elseif (is_object($choice)) {
      $options .= form_select_options($element, $choice->option);
    }
    else {
      $key = (string) $key;
      if ($value_valid && (!$value_is_array && (string) $element['#value'] === $key || ($value_is_array && in_array($key, $element['#value'])))) {
        $selected = ' selected="selected"';
      }
      else {
        $selected = '';
      }
      $options .= '<option value="' . check_plain($key) . '"' . $selected . '>' . check_plain($choice) . '</option>';
    }
  }
  return $options;
}

When a $choice is a Twig_Markup object, this line is totally wrong:

     $options .= form_select_options($element, $choice->option); 

Because Twig_Markup doesn't have an 'option' element. (This line is the same way in D7... what is ->option for?) This causes the recursion to happen with $choice being passed as NULL, which causes infinite recursion.