Transform stream `readable_get` and `writable_get` assertion failures

In the WPT debug build, `readable_get` and `writable_get` call `readable` and `writable` respectively, which fail an assertion when looking like:

```c++
JSObject *TransformStream::readable(JSObject *self) {
MOZ_ASSERT(is_instance(self));
return &JS::GetReservedSlot(self, TransformStream::Slots::Readable).toObject();
}
```

that the slot fails `toObject()`. In https://github.com/bytecodealliance/StarlingMonkey/pull/113 an explicit check is added, but this only hides the existing bug which still needs to be resolved.