~ avoid dup server#close

Author: huzhou

lib/process.js

@@ -473,7 +473,7 @@ Process.prototype.listen = function() {
                 log('[graceful] app status:%s', app.listening);
                 if(app.listening){
                     try {
-                        if(app.server){
+                        if(app.server && app.server._handle){
                             log('[graceful] app server shutdown');
                             app.server.close();
                         }