Fix audit test module initialization

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

https://github.com/krb5/krb5/commit/5036f91e7b61a73a1ec2d39ce1cc6bbf60dd82ab
Author: Greg Hudson <ghudson@mit.edu>
Commit: 5036f91e7b61a73a1ec2d39ce1cc6bbf60dd82ab
Branch: master
 src/plugins/audit/test/au_test.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)