Bug 15448: Fix ISE

Bug 15448: Fix ISE

The method Koha::Items->itemnumber is not covered by tests!

Trace begun at /kohadevbox/koha/Koha/Objects.pm line 592
Koha::Objects::AUTOLOAD('Koha::Items=HASH(0x55981fd94790)') called at /kohadevbox/koha/opac/opac-reserve.pl line 465

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 9790b5ca3a1b8a92d56a14207d6245c02dfe7fd6)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 60e08d51bb814eb1dae834afa0c259539eaf2c61)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>