Don't flush if the service was already paused

Fixes #87

Author: fubar-coder

src/FubarDev.FtpServer/Networking/PassThroughService.cs

@@ -42,8 +42,13 @@ public override async Task StopAsync(CancellationToken cancellationToken)
         {
             await base.StopAsync(cancellationToken)
                .ConfigureAwait(false);
-            await SafeFlushAsync(cancellationToken)
-               .ConfigureAwait(false);
+
+            if (!IsPauseRequested)
+            {
+                await SafeFlushAsync(cancellationToken)
+                   .ConfigureAwait(false);
+            }
+
             await OnCloseAsync(_exception, cancellationToken)
                .ConfigureAwait(false);
         }