Bug 14846 - Items with no holdingbranch causes svc/holds to crash

Bug 14846 - Items with no holdingbranch causes svc/holds to crash

When trying to display the holds for a user, if an item has no
holdingbranch, it causes the svc/holds service to crash and the display
to freeze.

Tentative steps to reproduce on any DB:
1) put 1 or more items on hold for a given user.
2) go into the database and set the item.holdingbranch to NULL
3) In the user's page, in checkouts (circulation.pl), click the tab that
   should be written 1 Hold(s)
4) It will be "Processing" indefinitely.

The cause is the svc/holds code that crashes.  The fix will validate
that there's an holdingbranch before proceding with the code.

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit c6f7496422a593fdfe4dd2633aed4b190703d03d)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>