The NSInputStream's init: selector is actually required (if a class inherits from NSInputStream). Fixes #9733.

There are two cases:

* A plain NSInputStream. In this case init: crashes and initWithData: works.
* A subclassed NSInputStream. In this case init: works and initWithData: throws a selector not found exception.

Since both cases happen in real code, we need both methods, even
if one of them crashes in each scenario.