Bug 18443: Get rid of warning 'uninitialized value $user' in C4/Auth.pm

Bug 18443: Get rid of warning 'uninitialized value $user' in C4/Auth.pm

When logging out from OPAC, plack-error.log log and/or opac-error.log
complain about 'uninitialized value $user' in C4/Auth.pm line 187. The
warning is not necessary, this patch removes it.

To test:
- try to trigger warning
- apply patch
- verify that warning no longer occurs
- prove t/db_dependent/Auth.t
- verifiy that SCO still behaves like before (especially if
  you break out from sco path)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>