[Koha] overdue notice lists all checked out books

[Yuval Hager]

Hi,

I noticed this after upgrading to 3.18.1 (from 3.16). If a patron has overdue
items, they receive the overdue notice, but the list of overdue items includes
all the books they have checked out, not only the overdue ones.

I also get this error on the cron run, not sure if it's related:
Use of uninitialized value $borrowernumber in string eq at
/usr/share/koha/bin/cronjobs/overdue_notices.pl line 544.

Installed on Debian from the koha repositories.