Bug 14534: Stop overdue_notices.pl from issuing the warning: Use of uninitialized...

Bug 14534: Stop overdue_notices.pl from issuing the warning: Use of uninitialized value $borrowernumber

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
To test:

1/ Run the overdue_notices.pl script (don't do this on production
obviously :))
2/ Notice the warns
3/ Apply patch
4/ Run again
5/ Notice no warns, but notices are still generated ok

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>