Fix #20134: Don't update grass length outside of the actual map

src/openrct2/world/Map.cpp

@@ -1335,6 +1335,9 @@ void MapUpdateTiles()
             for (int32_t blockX = 0; blockX < gMapSize.x; blockX += 256)
             {
                 auto mapPos = TileCoordsXY{ blockX + x, blockY + y }.ToCoordsXY();
+                if (MapIsEdge(mapPos))
+                    continue;
+
                 auto* surfaceElement = MapGetSurfaceElementAt(mapPos);
                 if (surfaceElement != nullptr)
                 {