Merge pull request #628 from AArnott/fixNREOnDisposal

Fix NRE thrown from Channel.Dispose in concurrent conditions

Author: AArnott

src/Nerdbank.Streams/MultiplexingStream.Channel.cs

@@ -381,7 +381,7 @@ public void Dispose()
                         // We can't know whether there is or will be more bytes written to the user's PipeWriter,
                         // but we need to terminate our reader for their writer as part of reclaiming resources.
                         // Cancel the pending or next read operation so the reader loop will immediately notice and shutdown.
-                        this.mxStreamIOReader.CancelPendingRead();
+                        this.mxStreamIOReader?.CancelPendingRead();
 
                         // Only Complete the reader if our async reader doesn't own it to avoid thread-safety bugs.
                         PipeReader? mxStreamIOReader = null;