Fixed loss of chain links in ngx_event_pipe_read_upstream().

src/event/ngx_event_pipe.c

diff --git a/src/event/ngx_event_pipe.c b/src/event/ngx_event_pipe.c
index fb00bcc..4f45bb8 100644
--- a/src/event/ngx_event_pipe.c
+++ b/src/event/ngx_event_pipe.c
@@ -409,6 +409,7 @@
             }
 
             p->free_raw_bufs = cl->next;
+            ngx_free_chain(p->pool, cl);
         }
     }