Add check for no width or height in previewImageLoaded

[srt4rulez]

Fixes #44 by returning early if images have no width or height in previewImageLoaded.

There's probably a better solution to this that can somehow figure out the width and height from the viewbox attribute, but this at least allows the image to continue uploading.

[rikschennink]

Thanks, I'll try to look into this as soon as possible but I'm swamped with other work right now, hope to have some more time in february.