Error content length when readstream have start and end options

I create a readstream with start and end options，the getLength get a error size

```
fs.createReadStream('sample.txt', {start: 90, end: 99});
```

lib/form_data.js line 82

```
      fs.stat(value.path, function(err, stat) {
        if (err) {
          next(err);
          return;
        }

        next(null, stat.size);
      });  
```
