The fix is simple and doesn't hurt any of your RSS subscriptions. Just quit Safari, remove the Database3 file located in ~/Library » Syndication, and then launch Safari again. I move mine to the Desktop and trash it only after I've successfully restarted Safari and confirmed the problem is fixed. Safari will create a new file in its place.
Easy enough, but I've never seen it anywhere else online, and have forgotten it myself a few times.

