Fixed annoying port of undefined exception during cluster startup. The problem was in port check that only master should do

Author: dimichgh

lib/index.js

@@ -184,12 +184,17 @@ Cluster.prototype.listen = function(createApp, cb) {
                 process.exit(-1);
             }
         });
-        server.listen(port[index], host, function() { //'listening' listener
-            exitIfBusyPort(host, port, index-1, function(){
-                server.close();
-                cb();
-            })
-        });
+        if (require('cluster').isMaster) {
+            server.listen(port[index], host, function() { //'listening' listener
+                exitIfBusyPort(host, port, index-1, function(){
+                    server.close();
+                    cb();
+                })
+            });
+        }
+        else {
+            process.nextTick(cb);
+        }
     }
 
     return self;