[PATCH] Core: slight optimization in ngx_chain_update_chains()

[胡聪 (hucc)]

Hi,

I found that sometimes the *out is NULL when i was reading the code
of ngx_event_pipe.c .  So it is not necessary to traverse *busy when
*out == NULL in ngx_chain_update_chains().

# HG changeset patch
# User hucongcong <hucong.c at foxmail.com>
# Date 1478744273 -28800
#      Thu Nov 10 10:17:53 2016 +0800
# Node ID c7b6269faec0d2ee6b4f4f625db8c219b5d0b010
# Parent  92ad1c92bcf93310bf59447dd581cac37af87adb
Core: slight optimization in ngx_chain_update_chains().

It is not necessary to traverse *busy and link the *out
when *out == NULL.

diff -r 92ad1c92bcf9 -r c7b6269faec0 src/core/ngx_buf.c
--- a/src/core/ngx_buf.c	Fri Nov 04 19:12:19 2016 +0300
+++ b/src/core/ngx_buf.c	Thu Nov 10 10:17:53 2016 +0800
@@ -189,7 +189,7 @@ ngx_chain_update_chains(ngx_pool_t *p, n
     if (*busy == NULL) {
         *busy = *out;
 
-    } else {
+    } else if (*out) {
         for (cl = *busy; cl->next; cl = cl->next) { /* void */ }
 
         cl->next = *out;