kadmin/version4.c doesn't check krb5_425_conv_principal

[Derrick J Brashear <shadow@dementia.org>]

in kadmin/version4.c krb5_425_conv_principal is called but the return
value isn't checked. in this case it's HEIM_ERR_V4_PRINC_NO_CONV, which
means the
krb5_unparse_name(context, client, &client_str); 
on the next line causes a SEGV.

-D