Bug 29915: Prevent bad cookie from corrupted session

Bug 29915: Prevent bad cookie from corrupted session

If there is deleted session info but no session->id, a wrong cookie
with empty name could be generated containing expired session id.

Test plan:
Run t/db_dependent/Auth.t
Login. Check cookies in browser.
Logout. Check cookies in browser.
Without this patch, you should see an invalid cookie.

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