Bug 20031: CGI param in list context warn in guided_reports.pl

Bug 20031: CGI param in list context warn in guided_reports.pl

Resolve warn:
CGI::param called in list context from package CGI::Compile::ROOT::usr_share_koha_shadowclone_reports_guided_reports_2epl line 794, this can lead to vulnerabilities. [NOTE: line number from older Koha version.]

Test plan:
Looking at this change should convince you already. Another look at this
routine in C4/Output.pm would be bonus.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>