[ticket/10605] Fix left join usage.

Previously this produced broken SQL:

SELECT p.msg_id FROM phpbb_privmsgs p
LEFT JOIN 0 phpbb_privmsgs_to ON (p.msg_id = t.msg_id)
WHERE t.user_id IS NULL LIMIT 500 OFFSET 0

PHPBB3-10605

phpBB/install/database_update.php

@@ -2039,7 +2039,7 @@ function change_database_data(&$no_updates, $version)
 				),
 				'LEFT_JOIN'	=> array(
 					array(
-						'FROM'	=> array(PRIVMSGS_TO_TABLE, 't'),
+						'FROM'	=> array(PRIVMSGS_TO_TABLE => 't'),
 						'ON'	=> 'p.msg_id = t.msg_id',
 					),
 				),