Merge pull request #924 from AArnott/fix906

Avoid internal exception on timeout of AsyncSemaphore request that races with disposal

src/Microsoft.VisualStudio.Threading/AsyncSemaphore.cs

@@ -193,6 +193,7 @@ protected virtual void Dispose(bool disposing)
                             head.Value.Trigger.TrySetException(new ObjectDisposedException(this.GetType().FullName));
                             waitersCopy.Add(head.Value);
                             this.waiters.RemoveFirst();
+                            head.Value.Node = null;
                         }
                     }