Only include /etc/krb5.conf if it exists
Only include /etc/krb5.conf if it exists

Including an invalid krb5.conf led to failure to init a
krb5_context.

https://bugs.freedesktop.org/show_bug.cgi?id=57403

freedesktop-unofficial-mirror/realmd__adcli,freedesktop-unofficial-mirror/realmd__adcli