MYSQL Get Month before
Kindly try the following code :

SELECT 

            catalog.code, 
            catalog.supplier,
            catalog.created_at

       FROM catalog
        WHERE MONTH(catalog.created_at) = month(NOW()- INTERVAL 1 MONTH)
        AND YEAR(catalog.created_at) = year(NOW());


Tried and tested in SQLFiddle: http://sqlfiddle.com/#!9/2f8d3/1/0
Hope this helps !!
Regards.