logind: fail user creation is slice creation fails

If the slice wasn't set, logind would core dump later in the process:
systemd-logind[2672]: Assertion 's->user->slice' failed at ../src/login/logind-session.c:531, function session_start_scope().

https://bugzilla.redhat.com/show_bug.cgi?id=1182781
1 file changed