[whatwg/streams] tighten ReadableStreamReaderGenericRelease assertion (#466)

It seems this could be tightened:

```
2. Assert: reader@[[ownerReadableStream]]@[[reader]] is not undefined.
```

To something like:

```
2. Assert: reader@[[ownerReadableStream]]@[[reader]] is reader.
```

---
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/466