nss-systemd: pack pw_passwd result into supplied buffer

getpwnam_r() guarantees that the strings in the struct passwd that it
returns are pointers into the buffer allocated by the application and
passed to getpwnam_r(). This means applications may choose to modify the
strings in place, as long as the length of the strings is not increased.
So it's wrong for us to return a static string here, we really do have
to copy it into the application-provided buffer like we do for all the
other strings.

This is only a theoretical problem since it would be very weird for an
application to modify the pw_passwd field, but I spotted this when
investigating a similar crash caused by glib editing a different field.
See also:

https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2244