Bug 28644: Fix calling borrowernumber on undefined value

Bug 28644: Fix calling borrowernumber on undefined value

If the hold is not found (e.g. already cancelled), we should
return earlier without crashing:
    Can't call method "borrowernumber" on an undefined value at /usr/share/koha/C4/Reserves.pm line 521
    (Note: line number from 19.11)

Test plan:
Run t/db_dependent/Reserves.t
Add a hold, go to user menu with holds in OPAC.
At the same time, cancel this hold from staff.
Now click the Cancel in OPAC.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>