Bug 28383: Fix itemsearch when accessed from the login form

Bug 28383: Fix itemsearch when accessed from the login form

There are params here (credentials), we need to test for the existence of $format.

Test plan:
logout
access /cgi-bin/koha/catalogue/itemsearch.pl
Login
=> Without this patch you get a 500 (because we hit the exit statement)
=> With this patch you see the items search form.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>