fix the lighthouse crash in preview.

scripts/eressea/xmasitems.lua

@@ -99,8 +99,6 @@ function self.update()
                     r:set_resource("tree", trees * 1.1)
                     msg:send_region(r)
                 end
-                if clear then
-                end
             end
         end
     else

src/kernel/save.c

@@ -1493,7 +1493,9 @@ int read_game(gamedata *data)
         if (r->flags & RF_LIGHTHOUSE) {
             building *b;
             for (b = r->buildings; b; b = b->next) {
-                update_lighthouse(b);
+                if (is_lighthouse(b->type)) {
+                    update_lighthouse(b);
+                }
             }
         }
     }