I think the rules should be changed a little to more closely reflect what I think is our real mission.
Code Review is really about refactoring--improving code without changing its (externally visible) behavior.
If code is so badly broken that its behavior clearly needs to change before it stands any chance of working at all, then refactoring is basically pointless.
In real life, however, it's fairly routine to decide that code with known bugs is worth refactoring to at least some degree. If that clarifies/simplifies the code to the point that some of the bugs are easy to fix, doing so is perfectly acceptable--but (for example) those bugs may be sufficiently minor in real use that making the code more readable and maintainable is a higher priority than fixing bugs.
That leaves something of a judgement call, but (IMO) generally a simpler one than we're faced with today. In particular, rather than being a judgement call about whether the code is bug-free to the best of the OP's knowledge, it becomes a judgement call about the intent of the question being asked. If the question is really about fixing the code, then it belongs on StackOverflow. If it's really about refactoring the code, then it can be a perfectly decent fit for CR, even if it has outstanding bugs.