Correct some "bad query" in the admin products getList

Here the wrong Sql generated :

SELECT SQL_CALC_FOUND_ROWS a.`id_product`,b.name as name,`reference`,a.price as price,a.active as active , MAX(i.id_image) id_image,cl.name `name_category` , a.`price`, 0 AS price_final, sav.`quantity` as sav_quantity, a.`active` FROM `ps_product` a LEFT JOIN `ps_product_lang` b ON (b.`id_product` = a.`id_product` AND b.`id_lang` = 2 AND b.`id_shop` = 1) LEFT JOIN `ps_image` i ON (i.`id_product` = a.`id_product` AND i.cover=1)LEFT JOIN `ps_category_lang` cl ON (a.`id_category_default` = cl.`id_category` AND b.`id_lang` = cl.`id_lang` AND cl.id_shop = 1) LEFT JOIN `ps_stock_available` sav ON (sav.`id_product` = a.`id_product` AND sav.`id_product_attribute` = 0 AND sav.id_shop = 1 ) WHERE 1 AND a.`active` = 0 GROUP BY a.id_product ORDER BY quantity desc LIMIT 0,50 

'quantity' is ambigus (for the ORDER BY)
Merge pull request #837 from axometeam/patch-21

[-] BO : bad query when filtering on status + sorting by quantity in the AdminProducts getList