Sessions may not exist. Address #924 for MySQL

We still need to audit / do something similar for the other DBs

raddb/mods-config/sql/counter/mysql/expire_on_login.conf

@@ -1,5 +1,5 @@
 query = "\
-	SELECT TIMESTAMPDIFF(SECOND, acctstarttime, NOW()) \
+	SELECT IFNULL( MAX(TIME_TO_SEC(TIMEDIFF(NOW(), acctstarttime))),0) \
 	FROM radacct \
 	WHERE UserName='%{${key}}' \
 	ORDER BY acctstarttime \