typo in variable in get_bookmark()

[DD32]

from wp-includes/bookmark.php:

	$link = $wpdb->get_row($wpdb->prepare("SELECT * FROM $wpdb->links WHERE link_id = %d LIMIT 1", $bookmark_id));
	$link->link_category = array_unique( wp_get_object_terms($link_id, 'link_category', 'fields=ids') );

$link_id is undefined, I'm assuming this is supposed to be $link->link_id

As a result of this typo, Categories for links were not being returned with get_bookark()

[ryan]

(In [6353]) Use correct var. Props DD32. fixes #5419