componentWillUnmount should only ever be called once

[jimfb]

Error boundries might help, if someone is using them, but if you're not using error boundaries we shouldn't ever attempt to unmount a single component instance more than once.

https://jsfiddle.net/fxq44m0t/

The only reason it's not an infinite loop is that we eventually run out of stack space :P. With good tail recursion in the JSVM, this could become an infinite loop.

Anyway, this is more than a contrived example, because the ReactRouter+redux combination creates situations where this type of thing happens.

[jimfb]

Fixed in #6613