sd-login: always return two arrays of same length from sd_seat_get_sessions

sd-login: always return two arrays of same length from sd_seat_get_sessions

sd_seat_get_sessions returns two arrays, that in principle should always match:
the session names and corresponding uids. The second array could be shorter only
if parsing or uid conversion fails. But in that case there is no way to tell
*which* uid is wrong, so they are *all* useless. It's better to simplify things and
just return an error if parsing fails.