admin: correct category sort below top category

[torvista]

Mentioned here: https://www.zen-cart.com/showthread.php?229613-category_product_listing-issue&p=1396101#post1396101

When in the subcategory immediately below the TOP, the sorting options redirect to the TOP category due to this:

if (isset($_GET['cID'])) {
  $_GET['cID'] = (int)$_GET['cID'];
}

$_GET['cID'] is set, but an empty string, but there is no category 0 so it redirects to the top, maintaining the previous sort order.

So a change to

if (!empty($_GET['cID'])) {
  $_GET['cID'] = (int)$_GET['cID'];

seems a fix: both category and product sort as selected.