Upstream: fixed $upstream_response_length without buffering.

Reported by Piotr Sikora.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index a7e88b7..45e2eb7 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3307,7 +3307,7 @@
         u->state->response_sec = tp->sec - u->state->response_sec;
         u->state->response_msec = tp->msec - u->state->response_msec;
 
-        if (u->pipe) {
+        if (u->pipe && u->pipe->read_length) {
             u->state->response_length = u->pipe->read_length;
         }
     }