Core: slight optimization in ngx_chain_update_chains().

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

src/core/ngx_buf.c

@@ -186,6 +186,7 @@ ngx_chain_update_chains(ngx_pool_t *p, ngx_chain_t **free, ngx_chain_t **busy,
 {
     ngx_chain_t  *cl;
 
+    if (*out) {
         if (*busy == NULL) {
             *busy = *out;
 
@@ -196,6 +197,7 @@ ngx_chain_update_chains(ngx_pool_t *p, ngx_chain_t **free, ngx_chain_t **busy,
         }
 
         *out = NULL;
+    }
 
     while (*busy) {
         cl = *busy;