MEDIUM: conn-stream: Be prepared to fail to attach a cs to a mux

MEDIUM: conn-stream: Be prepared to fail to attach a cs to a mux

To be able to move wait_event from the stream-interface to the conn-stream,
we must be prepare to handle errors when a mux is attached to a conn-stream.
Indeed, the wait_event's tasklet will be allocated when both a mux and a
stream will be both attached to a stream. So, we must be prepared to handle
allocation errors.