Change assert to return invalid roadtype identifiers instead

gistfile1.txt

diff --git a/src/road_map.h b/src/road_map.h
index 544083e..cf2d75d 100644
--- a/src/road_map.h
+++ b/src/road_map.h
@@ -562,9 +562,10 @@ struct RoadTypeIdentifiers {
 	 */
 	static RoadTypeIdentifiers FromTile(TileIndex t)
 	{
-		assert(IsTileType(t, MP_ROAD) || IsTileType(t, MP_STATION) || IsTileType(t, MP_TUNNELBRIDGE));
-		TileType tt = GetTileType(t);
 		RoadTypeIdentifiers rtids;
+		if (!(IsTileType(t, MP_ROAD) || IsTileType(t, MP_STATION) || IsTileType(t, MP_TUNNELBRIDGE))) return rtids;
+
+		TileType tt = GetTileType(t);
 		rtids.road_identifier = RoadTypeIdentifier();
 		rtids.tram_identifier = RoadTypeIdentifier();