authpf: do not sprintf to a null pointer

The fgetln loop will terminate with buf = NULL at EOF.

Reported by: GCC
Reviewed by: kp
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39947

(cherry picked from commit 81f964e2ffb7cd5d30df0a5c3877900cca4fdd07)