Bug 29840: Deal with non-existing biblionumber in opac-reserve

Bug 29840: Deal with non-existing biblionumber in opac-reserve

We should not explode if the biblionumber passed in not in DB.

Test plan:
Hit /cgi-bin/koha/opac-reserve.pl?biblionumber=42424
and notice the error message.

Without this patch you got
Can't call method "holds" on an undefined value at
/kohadevbox/koha/opac/opac-reserve.pl line 172

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>