Fix undefined variable in DPL\Query#_maxrevisions

This was resulting in invalid SQL like this being generated when certain options were used:

SELECT DISTINCT `page`.page_namespace AS `page_namespace`,`page`.page_id AS `page_id`,`page`.page_title AS `page_title` FROM `page` LEFT OUTER JOIN `categorylinks` `ecl1` ON ((`page`.page_id = ecl1.cl_from AND ecl1.cl_to ='Uncyclopedia_In-Jokes')) LEFT OUTER JOIN `categorylinks` `ecl2` ON ((`page`.page_id = ecl2.cl_from AND ecl2.cl_to ='Disambiguation_pages')) WHERE `page`.page_is_redirect = '0' AND `page`.page_namespace = '0' AND ecl1.cl_to IS NULL AND ecl2.cl_to IS NULL AND (`page`.page_id NOT IN (SELECT `templatelinks`.tl_from FROM `templatelinks` WHERE ((`templatelinks`.tl_namespace=10 AND `templatelinks`.tl_title='FFW')))) AND (NOT (`page`.page_title like'%/%' OR `page`.page_title like'UnPoetia:%' OR `page`.page_title like'UnReviews:%' OR `page`.page_title like'Unreviews:%')) AND (0 <= (SELECT count(*) FROM `categorylinks` WHERE `categorylinks`.cl_from=page_id)) AND (2 >= (SELECT count(*) FROM `categorylinks` WHERE `categorylinks`.cl_from=page_id)) AND (((SELECT count(rev_aux3.rev_page) FROM `revision` AS rev_aux3 WHERE rev_aux3.rev_page = `page`.page_id) <= )) ORDER BY page_len ASC LIMIT 500