update-utmp: fix assertion failure if rescue.target, multi-user.target and graphical...

update-utmp: fix assertion failure if rescue.target, multi-user.target and graphical.target are all inactive

If rescue.target, multi-user.target and graphical.target are all
inactive, get_current_runlevel() is not able to determine current
runlevel, and returns with zero. This zero runlevel value results to
assertion failure in utmp_put_runlevel().

 # systemctl stop rescue.target multi-user.target graphical.target
 # systemctl start systemd-update-utmp-runlevel.service

 systemd[1]: Stopped target Graphical Interface.
 systemd[1]: Stopped target Multi-User System.
 systemd[1]: Starting Update UTMP about System Runlevel Changes...
 systemd-update-utmp[67]: Assertion 'runlevel > 0' failed at src/shared/utmp-wtmp.c:275, function utmp_put_runlevel(). Aborting.
 systemd[1]: systemd-update-utmp-runlevel.service: Main process exited, code=dumped, status=6/ABRT
 systemd[1]: systemd-update-utmp-runlevel.service: Failed with result 'core-dump'.
 systemd[1]: Failed to start Update UTMP about System Runlevel Changes.

Let's just print a warning in this case and skip the utmp update, to
avoid systemd-update-utmp-runlevel.service failures.