Fix kadm5/gssrpc XDR double free [CVE-2014-9421]

[MITKRB5-SA-2015-001] In auth_gssapi_unwrap_data(), do not free
partial deserialization results upon failure to deserialize.  This
responsibility belongs to the callers, svctcp_getargs() and
svcudp_getargs(); doing it in the unwrap function results in freeing
the results twice.

In xdr_krb5_tl_data() and xdr_krb5_principal(), null out the pointers
we are freeing, as other XDR functions such as xdr_bytes() and
xdr_string().

ticket: 8056 (new)
target_version: 1.13.1
tags: pullup
xdr_dprinc_arg(XDR *xdrs, dprinc_arg *objp)
{
	if (!xdr_ui_4(xdrs, &objp->api_version)) {
		return (FALSE);
	}
	if (!xdr_krb5_principal(xdrs, &objp->princ)) {
		return (FALSE);
	}
	return (TRUE);
}

xdr_dprinc_arg(XDR *xdrs, dprinc_arg *objp)
{
	if (!xdr_ui_4(xdrs, &objp->api_version)) {
		return (FALSE);
	}
	if (!xdr_krb5_principal(xdrs, &objp->princ)) {
		return (FALSE);
	}
	return (TRUE);
}
