pam_xdg: Check asprintf return value
Reported by: Shawn Webb <shawn.webb@hardenedbsd.org>
Sponsored by: Beckhoff Automation GmbH & Co. KG
Fixes: 6e69612d5df1 ("pam: Add pam_xdg module")
(cherry picked from commit 2d2950c889335b24af7a92f3aaf9946de47bb0bc)