[D7] Value of last_render_text leaks into the next Dropbutton

Backport #2530634: Value of last_render_text leaks into the next Dropbutton.

This is the code I'm using at the moment.
Is $this->view->row_index equivilant to $values->index?

views_handler_field.inc

  class views_handler_field extends views_handler {
    ...
    public $additional_fields = array();

+   protected $last_render_index;
+
    /**
     * Construct a new field handler.
     */
    ...
    public function advanced_render($values) {
+     // Clean up values from previous render calls.
+     if ($this->last_render_index != $this->view->row_index) {
+       $this->last_render_text = '';
+     }
+
      ...
+     // If we rendered something, update the last render index.
+     if ((string) $this->last_render !== '') {
+       $this->last_render_index = $this->view->row_index;
+     }
+
      return $this->last_render;
    }
    ...
  }