Update upload/catalog/model/checkout/coupon.php

fix for 

f you insert a coupon and select some categories for it in admin when trying to use it doesnt work and returns error about the left column not existing in the category table.
in catalog/model/checkou/coupon.php
CODE: SELECT ALL
                  $coupon_category_query = $this->db->query("SELECT COUNT(*) AS total FROM `" . DB_PREFIX . "product_to_category` p2c LEFT JOIN `" . DB_PREFIX . "category` c ON (p2c.category_id = c.category_id) WHERE p2c.`product_id` = '" . (int)$product['product_id'] . "' AND c.`left` BETWEEN '" . (int)$category['left'] . "' AND '" . (int)$category['right'] . "'")




I guess its something unfisnished.