Update FrontController.php

Fixed the relation of Display products per page number when its modified on back office preferences>products>pagination: "Products displayed per page".

If you have 3 columns of products and 15 products per page, and you check the display list with 20 or 50 products, always we have a blank space at the end of the product list and you get the feeling of the list is finished. It's more easy get the var number and multiply with x pages, my recomendation 2, and 4, and then if you have and impar or par option you always have the list correctly filled