fix: PHP warning when remove items from cart when discount is applied on...

reported here
https://easydigitaldownloads.com/support/topic/discount-code-doesnt-seem-to-work/#post-165307

Replicated customer's setup with a subfolder install (normal install works fine), EDD 1.9.4 and WP 3.8.

When a discount is applied to cart, and you remove any item from the cart, it bombs out an invalid foreach, referencing the line with `wp_list_pluck` on it