Security: Avoid NULL structure pointer member dereference

This can happen in the error path when processing malformed AS
requests with a NULL client name.  Bug originally introduced on
Fri Feb 13 09:26:01 2015 +0100 in commit:

    a873e21d7c

    kdc: base _kdc_fast_mk_error() on krb5_mk_error_ext()

Original patch by Jeffrey Altman <jaltman@secure-endpoints.com>

kdc/kerberos5.c

@@ -2239,8 +2239,10 @@ out:
 				 &req->req_body,
 				 ret, r->e_text,
 				 r->server_princ,
-				 &r->client_princ->name,
-				 &r->client_princ->realm,
+				 r->client_princ ?
+                                     &r->client_princ->name : NULL,
+				 r->client_princ ?
+                                     &r->client_princ->realm : NULL,
 				 NULL, NULL,
 				 reply);
 	if (ret)