Bug 22675: Fix SCO behaviour on non-existent barcodes

Bug 22675: Fix SCO behaviour on non-existent barcodes

Bug 21206 replaced GetItem for Koha::Items->find, and introduced this
problem. This patch verifies $item is defined before attempting to use
its methods.

To test:
- Log into SCO
- Type an invalid barcode
=> FAIL: Things explode :-/
- Apply this patch, reload
- Type an invalid barcode
=> SUCCESS: A nice error message is displayed :-D
- Sign off :-D

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>