Fix null reference exception in UnpackV1.Unpack.Dispose()

Co-authored-by: adamhathcock <527620+adamhathcock@users.noreply.github.com>

src/SharpCompress/Compressors/Rar/UnpackV1/Unpack.cs

@@ -27,7 +27,7 @@ internal sealed partial class Unpack : BitInput, IRarUnpack
         if (!disposed)
         {
             base.Dispose();
-            if (!externalWindow)
+            if (!externalWindow && window is not null)
             {
                 ArrayPool<byte>.Shared.Return(window);
                 window = null;