pid1: fix segv triggered by status query (#26279)

pid1: fix segv triggered by status query (#26279)

If any query makes it to the end of install_info_follow() then I think symlink_target is set to NULL.
If that is followed by -EXDEV from unit_file_load_or_readlink(), then that causes basename(NULL)
which segfaults pid 1.

This is triggered by eg. "systemctl status crond" in RHEL9 if

/etc/systemd/system/crond.service
  -> /ram/etc/systemd/system/crond.service
  -> /usr/lib/systemd/system/.crond.service.blah.blah
  -> /usr/lib/systemd/system/crond.service