Bug 24412: (follow-up) prevent request.pl from failing

Bug 24412: (follow-up) prevent request.pl from failing

When no desk is defined request.pl returns an internal server
error. Check a desk is defined before asking for reservation deskname.

Plan test:

1. log in with a library with no desk
2. check in a reserved book so it'll be switch to “waiting reserve”
3. go to the book notice and click the Holds tab (request.pl page)
4. boum
5. apply patch, restart plack
6. refresh page
7. now loading properly

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>