Fixed CPU hog while freeing hc->busy after e662cbf1b932 (1.11.11).

Reported by Richard Stanway,
http://mailman.nginx.org/pipermail/nginx/2017-March/053296.html.

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index 1788bf3..dd54910 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -2904,6 +2904,7 @@
             }
 
             cl->buf = b;
+            cl->next = NULL;
 
             hc->busy = cl;
             hc->nbusy = 1;