do not unlink unix domain socket file while online upgrade
diff --git a/src/core/ngx_connection.c b/src/core/ngx_connection.c
index b6246c4..6056c3f 100644
--- a/src/core/ngx_connection.c
+++ b/src/core/ngx_connection.c
@@ -627,7 +627,8 @@
 #if (NGX_HAVE_UNIX_DOMAIN)
 
         if (ls[i].sockaddr->sa_family == AF_UNIX
-            && ngx_process != NGX_PROCESS_WORKER)
+            && ngx_process != NGX_PROCESS_WORKER
+            && ngx_new_binary == 0)
         {
             u_char *name = ls[i].addr_text.data + sizeof("unix:") - 1;