pam_systemd_home: clean up sd-bus when called about something else's user

pam_systemd_home: clean up sd-bus when called about something else's user

acquire_home() takes a reference to a sd-bus object, which the open_session
hook cleans on success. But only when handling a user actually owned by homed,
it did not clean it up when skipping because it is being invoked on a system
user.
We need to be careful with sd-bus here as pam_sm_open_session is the last
hook before forking, and we want to clean up sd-bus before that happens, or
we'll have a broken reference (FDs are cloexec) in the child process, which
will then assert when attempting to close them, or leak the bus connection
which causes dbus to complain loudly:

 dbus-daemon[62]: [system] Connection has not authenticated soon enough, closing it (auth_timeout=30000ms, elapsed: 30020ms)