No delivery pane + taxes = js errors

I noticed this issue because uc_taxes.js was generating js errors, in turn causing collapsible fieldsets not to work properly, rendering the rest of the checkout interface inoperative.

In uc_taxes.js, if you are dealing with a non-shippable product, the "delivery" pane may not show up, which means the following will get a null value:

  var s_f_name = $("input[@name*=delivery_first_name]").val();

And this causes a failure when this var was referenced later on line 76.

Best solution is probably to backport some of the updates from us_taxes.js that are in HEAD? That or making sure the variables are set to 0 if the form elements are not present.