Fix #9407: desync when founding a town nearby a station

"stations_near" wasn't updated when founding a town near
a station. As this variable is not saved, any client joining
after the town is founded has a different value for
"stations_near", potentially causing desyncs.

src/town_cmd.cpp

@@ -2310,7 +2310,11 @@ static void MakeTownHouse(TileIndex t, Town *town, byte counter, byte stage, Hou
 	if (size & BUILDING_2_TILES_X)   ClearMakeHouseTile(t + TileDiffXY(1, 0), town, counter, stage, ++type, random_bits);
 	if (size & BUILDING_HAS_4_TILES) ClearMakeHouseTile(t + TileDiffXY(1, 1), town, counter, stage, ++type, random_bits);
 
-	if (!_generating_world) {
+	/* Skip this somewhat expensive loop if we are on a map without any
+	 * station. Read: a new map that is being generated.
+	 * Note: we cannot use _generating_world, as it is also set when
+	 * CmdFoundTown is used. */
+	if (_station_pool.items != 0) {
 		ForAllStationsAroundTiles(TileArea(t, (size & BUILDING_2_TILES_X) ? 2 : 1, (size & BUILDING_2_TILES_Y) ? 2 : 1), [town](Station *st, TileIndex tile) {
 			town->stations_near.insert(st);
 			return true;