Fixed processing of application response in router.

If the first buffer contained only header and there were other buffers in chain,
those buffers were not processed.  In particular, this broke Perl application
responses with empty body.

src/nxt_router.c

@@ -2677,7 +2677,10 @@ nxt_router_response_ready_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg,
             nxt_work_queue_add(&task->thread->engine->fast_work_queue,
                                b->completion_handler, task, b, b->parent);
 
-        } else {
+            b = b->next;
+        }
+
+        if (b != NULL) {
             nxt_buf_chain_add(&r->out, b);
         }