uc_taxes should check for valid node before attempting to alter cart item

In uc_taxes_uc_cart_view_form_alter() and uc_taxes_uc_cart_item() the price of an item in the cart is modified to include taxes where applicable. Can there could first be a check to see if the item is a valid node?

I know that, strictly speaking, there should never be a non-node in the cart, but it's sometimes useful to show something in the cart other than a product, and there's no easy way to do this other than adding it as a "fake" cart item (this is the strategy used by uc_coupon). This currently produces some runtime "property of non-object" notices. The attached patch prevents this.