Starting multiplayer game with an unused player number crash the game with assertion error

[Hans Joachim Desserud]

To reproduce:
1. From main menu, select "Multi player" -> "LAN / Direct IP" -> "Host a new game".
2. Select "map" and pick a map for three players.
3. Change your role to player 3.
4. Click on "Change map" and select a map for only two players.
5. Notice you are still listed as player 3, and that player 1 and 2 are open.
6. Click on the open positions of player 1 and 2 to populate them with AIs.
7. Click on "Start game"

The game crash with the following error message:

 WARNING: There are 31 unloaded objects. This is a bug, please consider committing!
Registering script: (win_conditions,00_endless_game)
[Host]: Requesting sync reports for time 1
widelands: /home/user/widelands/src/wui/interactive_player.cc:144: int32_t Int_Player_overlay_callback_function(Widelands::TCoords<Widelands::FCoords>, void*, int32_t): Assertion `static_cast<Interactive_Player const *>(data)->get_player()' failed.
Aborted

If a player has selected a position, and the map is changed to a smaller map where this position is not available, this should be handled in some way. Perhaps converting the player to a spectator?

Note: I am not sure if the warning in the error message is related or not, as it doesn't appear every time.

[SirVer (sirver)]

Released in build16-rc1