fix r2972, it caused "zero size buf" alert.

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index a0fcd71..f07472b 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -2715,6 +2715,7 @@
             b->last_buf = 1;
 
         } else {
+            b->sync = 1;
             b->last_in_chain = 1;
         }
     }