Bug 12243 - Records with invalid ISBNs can cause searches to fail

Some types of invalid ISBNs, when run throught C4::Koha::NormalizeISBN
can produce ISBN objects that contain invalid ISBNs. This can be
reproduced with an ISBN that has an invalid prefix, group code or
publisher code. An example ISBN would be "0788893777 (2 DVD 45th ed)".

When attempting to look up a record with such an ISBN, you will get an
error along the lines of: Can't call method "as_string" on an undefined
value

Instead of checking for the BAD_ISBN state, we should be checking for
the GOOD_ISBN state vi the method is_valid.

Test Plan:
1) Edit a record, add the following ISBN to your record: 0788893777 (2 DVD 45th ed)
2) When Koha redirects to the record, you should see the error message described
3) Apply this patch
4) Reload the page, you should now see your record

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>