[whatwg/streams] ReadableByteStreamControllerClose accesses first element of pendingPullIntos without checking array length (#470)

In ReadableByteStreamControllerClose step 5 and 6 are:

```
5. Let firstPendingPullInto be the first element of controller.[[pendingPullIntos]].
6. If ! ReadableStreamHasBYOBReader(stream) is true, controller.[[pendingPullIntos]]
    is not empty, and firstPendingPullInto.[[bytesFilled]] > 0.
```

It seems like step 5 could trigger a range error if pendingPullIntos is empty and has no first element.  Should we be checking the length of the List before trying to access the first element?

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