[BUG] reassigning signals crashes the game

[Mariculous]

What happened?
Reassigning signalboxes will crash the game.

James, now you know why you got disconnected (or even crashed, don't know) from stephenson-siemens. I just did a signalbox reassignment at that time.

How to reproduce?
- Open the attached save
- Select the signal reassignment tool
- Reassign signals from the signalbox at (25,25) to the signalbox at (22,25)
- Happy game crash

Edit: it does not matter if there actually is any signal reassigned or not. It will also crash when reassigning from the unused signalbox to the used one.

[jamespetts]

Thank you for the report: I believe that I have now managed to fix this.

[Mariculous]

Fixed.