pam_xdg: Fix user xdg runtime directory's descriptor leak
Approved by: emaste (mentor)
Fixes: 6e69612d5df1 ("pam: Add pam_xdg module")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
(cherry picked from commit 06986e899972ac3a127ab2ab46196672d0e1e5b2)
Approved by: emaste (mentor)