Do not call xdr_string() with a NULL error message

It is illegal to call xdr_string() with a NULL string. Linux
just retruns false, NetBSD gets a SIGSEGV when xdr_string()
calls strlen(NULL)

BUG: 916439
Change-Id: Ia958470ada6e8e55a86d439922ec942d038f5f13
Original-author: Emmanuel Dreyfus <manu@netbsd.org>
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/4629