tls: don't call gnutls_deinit() after failed gnutls_init()

Don't assume gnutls_init() leaves the session pointer at NULL when it
returns with an error status. It might be a session that was already
allocated and then freed without resetting it to NULL after an error.

Fixes: 3e32e7e694 ("tls: move gnutls code into tls_gnutls.c")

tls_gnutls.c

@@ -180,6 +180,7 @@ TLS_CreateInstance(int server_mode, int sock_fd, const char *server_name,
                                   (server_mode ? GNUTLS_SERVER : GNUTLS_CLIENT));
   if (r < 0) {
     LOG(LOGS_ERR, "Could not %s TLS session : %s", "create", gnutls_strerror(r));
+    inst->session = NULL;
     goto error;
   }