ignore reading jpeg thumbnail image with no content

Image::Info cannot work for the jpeg image which has meta information of thumbnail but does not contain thumbnail image data itself (attached example image). I'm not sure why such image exists. I just found the image by chance on the web. But maybe some exif editor removes the thumbnail data without cleaning meta information together, I guess.

My patch skips reading thumbnail image data if it is empty.

![invalid_jpeg](https://f.cloud.github.com/assets/25668/463326/29245e5e-b54d-11e2-91fa-669c317eecb8.jpg)

(I'm sorry I've send a wrong pull-req before this.)