Invalid reference missed.

Hi there,

First of all big thanks for your work. The validator helped me much in
perfecting my schema. However I found a little bug in it.
Included schemas valid and invalid differs by one line - the
definition of 'begin' element which is called by ref in 'log' group
definition. Of course 'begin' element must be defined but if it is not,
the validator does not report an error.
Hope you can fix it easily.

Grettings,
MoMaT