[18SJ] Add check to Step::AssignGotaKanalbolaget

When selling shares in emergency money raising,
Step::AssignGotaKanalbolaget.actions was being called with a player
entity, causing an error when `@game.abilities` was called. This adds a
check to make sure the entity is a corporation.

Fixes #9759.

lib/engine/game/g_18_sj/step/assign_gotakanalbolaget.rb

@@ -20,7 +20,7 @@ def description
           end
 
           def ability(entity)
-            return if !entity || !@game.gkb || @game.gkb.owner != entity
+            return if !entity || !entity.corporation? || !@game.gkb || @game.gkb.owner != entity
 
             ability = @game.abilities(entity, :assign_hexes)
             return if !ability || ability.count.zero?