Code review comment for lp://staging/~widelands-dev/widelands/bug-1647033-immovable-packet-no

I could create a stable version of Gold Rush (online as version 1.2) with this branch and it loads into B19. Noteworthy, the save games of later versions still report the same error when loaded in B19. Not necessarily a problem. Savegames probably will grow incompatible anyway soon, so it's not worth fixing this part.