Fix crash on logout/login via SunRay

Turns out this bug is introduced by commit
b4f893eb294883909759fc9fd825eade1a9f308c, which is later than
when themulti-seat branch was created.

This happens when user logout, c-k-d tries to activate the reserved
Login window session, and send out the two signals in
change_active_session().

The old_session is the session that user just log out, which should
be removed after logout. There is race condition here, if the
old_session get unrefed to 0 before the g_signal_emit call,
then a core dump happens. Reported in:
https://bugs.freedesktop.org/show_bug.cgi?id=28598