streams: calling end while ending will never invoke callback?

Calling `end()` twice with callback will cause the second callback to never be invoked:

e.g. the following will fail.

```js
writable.end(common.mustCall());
writable.end(common.mustCall());
```

I'm not sure what the behavior should be here. Maybe calling the callback with an error? Either way, not calling the callback at all seems to me like it will cause problems and memory leaks.