ReadableStreamDefaultControllerShouldCallPull implicitly assumes that
desiredSize is not null when it compares it to 0. This is guaranteed by
the check of
ReadableStreamDefaultControllerCanCloseOrEnqueue(controller), but it's
not obvious. Add an assert to make it explicit.

Also do the same for ReadableByteStreamControllerShouldCallPull.

No functional changes.

ReadableStreamDefaultControllerShouldCallPull was missing an exclamation mark
before ReadableStreamDefaultControllerGetDesiredSize. Fix it.