Bug 30099: Fix error when accessing circulation.pl without patron parameter

Bug 30099: Fix error when accessing circulation.pl without patron parameter

If I access circulation.pl directly, with no patron cardnumber or borrowernumber submitted, I get an error:
Can't call method "messages" on an undefined value at /kohadevbox/koha/circ/circulation.pl line 525

This is caused by Bug 29230 which calls $patron outside a
"if ($aptron)".

Test plan :
1) In staff interface, click in 'check out' search box
2) Hit enter (without a search text)
=> Without patch you get an error
=> With patch you see a blank page /cgi-bin/koha/circ/circulation.pl

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>