Bug 12829: properly skip invalid ISBN when rendering for normal record display

Bug 12829: properly skip invalid ISBN when rendering for normal record display

Currently, 020$z (and 010$z fr UNIMARC) occurences are not shown, but the XSLT logic
used introduces punctuation characters for those $z occurences.

This patch adds a check for the existence of subfield a, and only loops
on $a subfields.

To test:
- Create/have a record with 020$z (or 010$z on UNIMARC) but no 020$a (no 010$a on UNIMARC).
- Open the detail page for the record (on both OPAC and staff).
=> FAIL: the ISBN label and ';' and '.' characters incorrectly shown.
- Repeat mixing with 020$a occurences to notice the bug.
- Apply the patch, reload
=> SUCCES: ISBN label shows only on the presence of a $a, and $z are skipped.
           no matter how many ISBN fields you add.
- Sign off :-D

Regards
To+

http://bugs.koha-community.org/show_bug.cgi?id=12901
Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>