Bug 22321: Fix store() crash when modifying borrowernumber of a new ILL request

Bug 22321: Fix store() crash when modifying borrowernumber of a new ILL request

To test:
1. prove t/db_dependent/Illrequests.t

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Tried to change the borrowernumber connected to an ILL request
and checked that the associated hold was updated to the new
patron as well.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>