SVG: image cannot render when no source_width/height is available

We have some SVGs without width/height in the source code. This leads to a `ValueError` and fallback to `target_width/height` here: https://github.com/plone/plone.scale/blob/master/plone/scale/scale.py#L616

Unfortunately, `image.read()` returns an empty `b''` because `image.seek(0)` is missing before and this leads to a broken image.