Bug 27548: "use of uninitialized value" warning on branchoverdues.pl

Bug 27548: "use of uninitialized value" warning on branchoverdues.pl

On the beginning we have empty "location" parameter for this form,
but that not taken into account in comparison and undef warning emitted:
"Use of uninitialized value $location in string eq at
/usr/share/koha/intranet/cgi-bin/circ/branchoverdues.pl line 72" in branchoverdues.pl

Fixed by adding pre-check for $location.

To reproduce:
    1) Head over to "Circulation ->Overdues with fines" page. Check
       intranet-error.log where there will be many warnings
       "Use of uninitialized value $location in string eq at /usr/share/koha/intranet/cgi-bin/circ/branchoverdues.pl line 72".
    2) Apply the patch, refresh the "Overdues with fines" page.
    3) Ensure that the new "use of uninitialized value" warnings didn't appear in the console.

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>