net: allow socket end before connect

Fix a bug where calling .end() on a socket without calling .connect() first
throws a TypeError:

  TypeError: Cannot read property 'shutdown' of undefined
      at Socket.onSocketFinish (net.js:194:20)
      at Socket.EventEmitter.emit (events.js:91:17)
      at Socket.Writable.end (_stream_writable.js:281:10)
      at Socket.end (net.js:352:31)

Fixes #4463.

lib/net.js

@@ -191,7 +191,7 @@ function onSocketFinish() {
   debug('oSF: not ended, call shutdown()');
 
   // otherwise, just shutdown, or destroy() if not possible
-  if (!this._handle.shutdown)
+  if (!this._handle || !this._handle.shutdown)
     return this.destroy();
 
   var shutdownReq = this._handle.shutdown();