[0.12.19] [cube] If you merge a force with a disconnected player in it, crash

[Divran]

Steps to reproduce:
1. Start a server with two players (or a dedicated server with one player, as long as the server stays up after one person leaves).
2. Create a new force.
3. Join that force with one player.
4. That player disconnects.
5. Try to merge that force into any other force.
6. ???
7. ded

Bug discovered while testing my mod https://forums.factorio.com/forum/vie ... 94&t=18079
Last tested in version 0.12.18

[cube]

Thanks for the report, this is fixed for 0.12.20