Fix a memory leak in the built-in GnuTLS support.

Fix a memory leak in the built-in GnuTLS support.
* src/gnutls.c (emacs_gnutls_deinit): New function.  Deallocate
credentials structures as well as calling gnutls_deinit.
(Fgnutls_deinit, Fgnutls_boot): Use it.

* src/process.c (make_process): Initialize GnuTLS credentials to NULL.
(deactivate_process): Call emacs_gnutls_deinit.