Fix crash if bitmapStream is null. Fixes #10390 (#10428)

This fixes a regression introduced in 3854d32.

Author: bgrainger

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Imaging/BitmapDecoder.cs

@@ -1165,7 +1165,7 @@ out decoder
             }
             catch
             {
-                bitmapStream.Close();
+                bitmapStream?.Close();
 
                 decoderHandle = null;
                 throw;