Fix helpers/xmodel.php navigator mysql bug

There is a Bug in MySQL for Select count with Limit > 0

http://bugs.mysql.com/bug.php?id=50005
"LIMITing result set with a LIMIT clause >= 1 does nothing".

A workaround in MySQL could be (for instance) a subquery in the FROM-clause like

SELECT COUNT(*) FROM (SELECT * FROM the_table LIMIT y);