Prevent crashes from writing to closed data stream controllers

[hiroshihorie]

Changes:

• Add isClosed guards to write(), error(), and close() on DataStreamController
• Fix fallthrough bug in handleStreamChunk where a chunk would still be written after an encryption mismatch error
• Close stream controllers in validateParticipantHasNoActiveDataStreams after sending the error, so readAll() consumers don't hang indefinitely

Closes #978