Bug 7810 - C4/Auth.pm - on plack restart session is undefined

Bug 7810 - C4/Auth.pm - on plack restart session is undefined

So we shouldn't try to delete it. This produces application error
instead of redirection to login page.

I had similar problems with CGI, especially when session in browser
is still active, and one on filesystem or database is already expired.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Solved the problem when my Plack installation started acting up due to
stale cookies.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>