Don't show previous post link on orphaned attachments

[zeo]

In function adjacent_post_link(), add check if post parent not empty.

Test case.

For "back to parent" navigation, in theme template attachments.php or image.php, use:

<?php previous_post_link(); ?>

instead of the usual:

<a href="<?php echo get_permalink($post->post_parent); ?>" rev="attachment"><?php echo get_the_title($post->post_parent); ?></a>

or

if (!empty($post->post_parent))
...