Bug 37881: Editing patron with guarantor won't crash

Bug 37881: Editing patron with guarantor won't crash

This patch fixes issue introduced by BZ32530

Test Plan (on main):
1 - Edit a patron
2 - Do not change anything and submit -> you get an error
3 - Apply both patches
4 - Repeat 1&2 -> everything works fine
5 - Try and delete the guarantor -> it will be deleted

Signed-off-by: Brendan Lawlor <blawlor@clamsnet.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>