Typo in ErrorReadableStream algorithm

[youennf]

In the HTML draft currently available at https://streams.spec.whatwg.org/#error-readable-stream, step 6 is probably wrong. It should probably say something like: if reader is undefined, return undefined.
Step 6a could also be renumbered as step 7.

[domenic]

Thanks! I think I see the fix. But I'll let @tyoshino do it to confirm. This was introduced in the most recent commit 7b034bb

[tyoshino]

Thanks for review and catching this! Right, it's a bug. I've fixed the code as you suggested.