Bug 12198: (follow-up) fix warning when opening guarantor search form

Bug 12198: (follow-up) fix warning when opening guarantor search form

To test:

[1] Verify that act of running a guarantor search does not result in
    the following warning message in the Apache log:

"guarantor_search.pl: Use of uninitialized value $member in string ne
 at guarantor_search.pl line 63"

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>