Description of problem:
Originally reported at:
https://lists.gnupg.org/pipermail/gnutls-devel/2016-October/008209.html

... That said I also reproduced the issue; the fault is with the older
version, as it requires a CertificateStatus message if ServerHello
included the (empty) "status_request" extension, although RFC 6066
explicitly states:

   Note that a server MAY also choose not to send a "CertificateStatus"
   message, even if has received a "status_request" extension in the
   client hello message and has sent a "status_request" extension in the
   server hello message.

So the new gnutls code doesn't do anything wrong by replying with an
empty "status_request" extension, even if there is no chance of sending
a CertificateStatus message, just the old versions can't handle it.

The new version handles this by passing optional = 1 in the call to
_gnutls_recv_handshake(GNUTLS_HANDSHAKE_CERTIFICATE_STATUS) in
status_request.c.