As of r75968, SVG images that are invalid XML no longer render. Previously those images where allowed by MediaWiki and it appears that rsvg happily accepts them.
I agree that we should not accept SVG images which are not valid XML, however I am not convinced that breaking images which used to render fine is a good approach.
People in Commons suggest that a few hundreds of SVGs on Commons are broken. My analysis on a random set suggests that less than 0.1% of the SVG images in use on Wikimedia is affected. (Checked a random set of 1000 images, found none)
The question is whether we should fall back to the old behavior if the file does not pass the new file checks, or whether we should just refuse rendering them.