test for if child exists before replacing menu_slug

This solves an issue where top-level options pages don't appear. `api-options-page.php` looks for a child and then gives this to the parent as the menu slug. However, this fails and menus come-up blank when there's no menu slug:
![screen shot 2014-06-04 at 1 16 32 pm](https://cloud.githubusercontent.com/assets/1298086/3177127/4061509e-ec0c-11e3-8c61-7155be784d41.png)

This PR tests for the existence of the child slug before assigning it as the parent's menu slug.