Assertion and bugfix. (#397)

chronos/asyncfutures2.nim

@@ -222,6 +222,7 @@ proc finish(fut: FutureBase, state: FutureState) =
   # 1. `finish()` is a private procedure and `state` is under our control.
   # 2. `fut.state` is checked by `checkFinished()`.
   fut.state = state
+  doAssert fut.cancelcb == nil or state != FutureState.Cancelled
   fut.cancelcb = nil # release cancellation callback memory
   for item in fut.callbacks.mitems():
     if not(isNil(item.function)):

chronos/streams/asyncstream.nim

@@ -961,7 +961,7 @@ proc join*(rw: AsyncStreamRW): Future[void] =
 
   if not(rw.future.finished()):
     rw.future.addCallback(continuation, cast[pointer](retFuture))
-    rw.future.cancelCallback = cancellation
+    retFuture.cancelCallback = cancellation
   else:
     retFuture.complete()