'Update module' in module editor becomes sometimes unavailable after undo

[Lorenzo Marcantonio]

To reproduce (bzr pcbnew as today):

- Open a board with a module in it
- Open module editor
- Put a module in edit with the 'load module from current board' button
- Add a graphic line
- Click the undo button
-> the update module in board becomes gray (and stays that way...)

It also happened other times but this is 100% reproducible

[Martin Errenst (imp-d)]

r2395 stable