Bug 20088: Fix uninitialized warning from svc/holds

Bug 20088: Fix uninitialized warning from svc/holds

Use of uninitialized value in array element at svc/holds line 56.

Test plan:
Check if the warn is no longer in the logs; the warning is triggered by
an ajax call from the circulation template in staff.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>