In the process, make test_kill_pid() safer: do not try to terminate
more than one pid, or the NULL handle.
Also stop leaking the address_fd in spawn_dbus_daemon, a pre-existing
bug that was spotted by Philip Withnall during review.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=88810
Reviewed-by: Philip Withnall
