SSL: fixed possible segfault on renegotiation (ticket #845).

Skip SSL_CTX_set_tlsext_servername_callback in case of renegotiation.
Do nothing in SNI callback as in this case it will be supplied with
request in c->data which isn't expected and doesn't work this way.

This was broken by b40af2fd1c16 (1.9.6) with OpenSSL master branch and LibreSSL.

src/http/ngx_http_request.c

@@ -837,6 +837,10 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
 
     c = ngx_ssl_get_connection(ssl_conn);
 
+    if (c->ssl->renegotiation) {
+        return SSL_TLSEXT_ERR_NOACK;
+    }
+
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                    "SSL server name: \"%s\"", servername);