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
Security: FreeBSD-EN-17.pam_xdg
Approved by: so
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 06986e899972ac3a127ab2ab46196672d0e1e5b2)
(cherry picked from commit 9e8d504bb5a1e7ede55814069e213d4702172b81)