Crash when burning enemies military sites

When a medium military building is conquered, small military buildings very close to it burn down without a fight.
When multiple small military buildings are burned down at once, the game crashes due to a failed assertion (when releasing the area of the small sites):

 assert(preferred_player != player_area.player_number); (logic/editor_game_base.cc line 663).

The simplest fix seems to be to remove the assert(). It doesn't seems to be required for the correct functioning of the method and after removing it the game seems to work fine.
The linked branch applies this "fix". I haven't created a merge request yet since I am unsure whether we really want to fix it that way, but feel free to do so.