Bug 19074: Do not crash if cardnumber does not exist

Bug 19074: Do not crash if cardnumber does not exist

If a cardnumber does not exist, $borrower will be undef and the ->category call will explode
Can't call method "category" on an undefined value at
/home/vagrant/kohaclone/tools/modborrowers.pl line 370.

This patch makes sure the patron exists before calling any methods.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>