Error when upgrading with no links/bookmarks

[ryanscheuermann]

When upgrading using the latest trunk code, I get this error because I have no links in my current DB:

Warning: Invalid argument supplied for foreach() in wp-admin\upgrade-functions.php on line 525

It does a foreach without checking if $links is null:

		$links = $wpdb->get_results("SELECT link_id, link_category FROM $wpdb->links");
		foreach ( $links as $link ) {

(In [3734]) Make sure we have links to iterate over. fixes #2675