Bug 12782: t/db_dependent/XISBN.t fails in DOM setup

Bug 12782: t/db_dependent/XISBN.t fails in DOM setup

To test:
- Have a DOM setup
- Run
  $ prove -v t/db_dependent/XISBN.t
=> FAIL: Can't call method "field" on an undefined value at .... C4/Search.pm ...
- Apply the patch
- Run
  $ prove -v t/db_dependent/XISBN.t
=> SUCCESS: Tests pass.
- Sign off

Regards
To+

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>