Port gnutls.c to older (buggier?) GnuTLS

Port gnutls.c to older (buggier?) GnuTLS

Problem reported for GnuTLS 3.2.1 by Glenn Morris in:
http://lists.gnu.org/archive/html/emacs-devel/2017-07/msg00716.html
http://lists.gnu.org/archive/html/emacs-devel/2017-07/msg00742.html
Although I don't see how this bug can occur with vanilla GnuTLS 3.2.1,
perhaps hydra was using a modified GnuTLS.
* src/gnutls.c (Fgnutls_ciphers): Don't assume GNUTLS_CIPHER_NULL
is at the end of the list returned by gnutls_cipher_list,
or that the earlier ciphers all have non-null names.