Stream.Read should return as soon as any data is available

(cherry picked from commit dec0e3f8fab991fb343a3d0c6458d44bd0d1fea9)

Source commit: d100218e1638ccdd1675f4413d6c6a2811756522

dotnet/internal/PipeStream.cs

@@ -145,7 +145,7 @@ namespace WinSCP
 
             lock (_buffer)
             {
-                while (!_isDisposed && !ReadAvailable(count))
+                while (!_isDisposed && !ReadAvailable(1))
                 {
                     Monitor.Wait(_buffer);
                 }