app.render callbacks twice

`app.render` method with callback sometimes callbacks twice, if callback throws exception.

``` js
var app = express();
app.set('view engine', 'ejs');

var called = 0;
app.render(__dirname + '/template.ejs', function (err, body) {
  console.log('called: ', ++called);
  throw new Error('some error');
});
```

Express.js: 4.12.4
ejs: 2.3.1
Node.js: 0.12.2
