pam_ksu: Fix crash when no ticket is present

[ivy]

Summary

When building with MIT Kerberos, pam_ksu crashes if the user doesn't
have a ticket because default_principal is never populated in
get_su_principal().

Change the compatibility function to use krb5_build_principal_alloc_va
instead, and make its interface compatible with the equivalent Heimdal
function.

Despite what the comment says, we do free the default principal later
in get_su_principal() so this shouldn't cause any leaks.

[ivy]

remove unneeded prototype for krb5_build_principal_va

[des]

Note that we still have Heimdal and the option to use it, and I don't see a krb5_free_principal() call anywhere in your patch...

[ivy]

In D51829#1184019, @des wrote:

Note that we still have Heimdal and the option to use it, and I don't see a krb5_free_principal() call anywhere in your patch...

line 270 is:

	krb5_free_principal(context, default_principal);

i am not an expert on the krb5 API but this seems to do what we want.

[des]

ah so the comment is bogus...

[des]

I don't know if this works with Heimdal, but I have verified that it builds, so I think we're good to go.

[philip]

I tested this in the FreeBSD.org cluster and it works. Many thanks!