Bug 14015 - Checkout: Fix software error if barcode '0' is given

Bug 14015 - Checkout: Fix software error if barcode '0' is given

To reproduce the bug:
- Go to Home > Circulation > Checkouts > [a patron]
- Enter 0 (zero) in barcode field and hit Enter

Result:
Software error
  Can't use string ("1") as a HASH ref while "strict refs" in use
  at /usr/share/kohaclone/circ/circulation.pl line 473

To test:
- Apply patch
- Repeat steps above
- Verify that the sofware error is gone and that you get a
  message as with other invalid barcodes.
- Test with other values: '00', '000', existing barcode,
  and a barcode that does not exist
- Test with empty barcode (for printing slip)
- Search for regressions

This patch implements Jonathan's solution (see comment #15)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>