Bug 20083: (follow-up) use same logic in opac-showmarc

Bug 20083: (follow-up) use same logic in opac-showmarc

It was correctly pointed out that opac-showmarc would leak
the same way as catalogue/showmarc.pl, and so this patch
moves the authentication step up to the top where it
should be so as to prevent inappropriate data leaks.

TEST PLAN
---------
1) Set your OpacPublic system preference to Disabled
2) Open your OPAC and login
3) Find a biblio with items
4) Go to the opac details, particularly MARC view.
5) Copy the "view plain" shortcut link.
6) log out.
7) Paste the link into the address bar.
   -- the information will leak!
8) apply the patch
9) restart_all
10) Refresh the OPAC link
    -- log in screen will appear.
11) run koha qa test tools

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