Follow-up patch : use the subfield associated with items.barcode

Follow-up patch : use the subfield associated with items.barcode

Instead of using the 'p' subfield directly, use the subfield letter
associated with the items.barcode column.

http://bugs.koha-community.org/show_bug.cgi?id=6963
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Test cases tested:
- 1 item, barcode empty
- 2 items, barcodes both empty
- 1 item, existing barcode
- 1 item, new barcode
- 2 items, one barcode empty, one existing

All test cases worked nicely and gave correct error messages.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>