Removed redundant stream check

Author: sakno

src/DotNext.IO/IO/PoolingBufferedStream.cs

@@ -674,13 +674,13 @@ public override IAsyncResult BeginRead(byte[] data, int offset, int count, Async
     public override Task FlushAsync(CancellationToken token)
     {
         Task task;
-        if (writePosition > 0)
+        if (stream is null)
         {
-            task = WriteAndFlushAsync(token);
+            task = DisposedTask;
         }
-        else if (stream is null)
+        else if (writePosition > 0)
         {
-            task = DisposedTask;
+            task = WriteAndFlushAsync(token);
         }
         else
         {
@@ -697,11 +697,10 @@ public override Task FlushAsync(CancellationToken token)
 
     private async Task WriteAndFlushAsync(CancellationToken token)
     {
+        Debug.Assert(stream is not null);
         Debug.Assert(writePosition > 0);
         Debug.Assert(buffer.Length > 0);
 
-        ThrowIfDisposed();
-
         await stream.WriteAsync(WrittenMemory, token).ConfigureAwait(false);
         await stream.FlushAsync(token).ConfigureAwait(false);