fix cycle in list ifd, fix #161

[rom1504]

In some rare case (see #161), there can be a cycle in the ifd, resulting in an infinite loop when trying to extract the exif of an image
this fixes this problem

[rom1504]

I published this as https://pypi.org/project/ExifRead-nocycle/ ; if anyone wants the fix sooner they can use it

But of course I would rather see this merged and published!

[rom1504]

Thanks for merging!

Could you consider doing a new pypi release?