pam_xdg: Reset rv to 0

Otherwise on success it ontain the return value of asprintf.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Fixes: 2d2950c88933 ("pam_xdg: Check asprintf return value")

(cherry picked from commit 5acbe6d1fb552eeaf44955c2780f2e9d4a70cad3)