http: Check this.connection before using it

Refer: https://github.com/joyent/node/pull/25670

PR-URL: https://github.com/nodejs/io.js/pull/2172
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>

lib/_http_outgoing.js

@@ -571,7 +571,9 @@ OutgoingMessage.prototype.end = function(data, encoding, callback) {
   // There is the first message on the outgoing queue, and we've sent
   // everything to the socket.
   debug('outgoing message end.');
-  if (this.output.length === 0 && this.connection._httpMessage === this) {
+  if (this.output.length === 0 &&
+      this.connection &&
+      this.connection._httpMessage === this) {
     this._finish();
   }