Bug 16073: Fix software error in circulation.pl

Bug 16073: Fix software error in circulation.pl

Can't call method "guarantor" on an undefined value at
/home/koha/src/circ/circulation.pl line 588.
 at /home/koha/src/circ/circulation.pl line 673

Introduced by bug 15656

Test plan:
Just trigger circ/circulation.pl

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Triggered circ/circulation.pl - nothing exploded, and error is gone
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com