Fix audit test module initialization

Check if the fopen result is NULL, not whether it's less than zero.

(cherry picked from commit 5036f91e7b61a73a1ec2d39ce1cc6bbf60dd82ab)

https://github.com/krb5/krb5/commit/0b5ea2766879ec1e2e632c16e5219daeec599446
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 0b5ea2766879ec1e2e632c16e5219daeec599446
Branch: krb5-1.12
 src/plugins/audit/test/au_test.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)