HTTP/2: fixed possible buffer overrun (ticket #893).

Due to greater priority of the unary plus operator over the ternary operator
the expression didn't work as expected.  That might result in one byte less
allocation than needed for the HEADERS frame buffer.

src/http/v2/ngx_http_v2_filter_module.c

@@ -215,8 +215,8 @@ ngx_http_v2_header_filter(ngx_http_request_t *r)
     clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
 
     if (r->headers_out.server == NULL) {
-        len += 1 + clcf->server_tokens ? ngx_http_v2_literal_size(NGINX_VER)
-                                       : ngx_http_v2_literal_size("nginx");
+        len += 1 + (clcf->server_tokens ? ngx_http_v2_literal_size(NGINX_VER)
+                                        : ngx_http_v2_literal_size("nginx"));
     }
 
     if (r->headers_out.date == NULL) {