Quick fix for download-map progress bars to be more accurate (#12567)

Download length is currently always '-1', the negative values
cause odd behavior.

game-app/game-core/src/main/java/games/strategy/engine/framework/map/download/MapDownloadProgressListener.java

@@ -28,7 +28,7 @@ private void updateProgressBarWithPercentComplete(
  () -> {
  progressBar.setIndeterminate(false);
  progressBar.setString(null);
- progressBar.setValue(percentComplete);
+ progressBar.setValue(Math.max(0, percentComplete));
  progressBar.setToolTipText(toolTipText);
  });
  }