Truncated mpo takes forever

Image: http://doppelbauer.name/takesforever.jpg
(do you mind if I delete the image?)

Testcase:
````python
import PIL.Image, PIL.ImageFile
PIL.ImageFile.LOAD_TRUNCATED_IMAGES = True
with PIL.Image.open('takesforever.jpg') as img:
    for frame in PIL.ImageSequence.Iterator(img):
        frame.convert('RGBA')
````