Fix dangling-callback in "early return" example code

This came up [here](https://github.com/nodeschool/discussions/issues/115#issuecomment-30554909), raised by @mikeal.

I'm a little weary of making this example more complex than it is because it's only trying to describe a very simple concept. Could I get some feedback from other maintainers about this before it gets merged (or not merged) please?