Disallow converting town-owned roads to types with the no houses flag

src/road_cmd.cpp

@@ -2966,6 +2966,13 @@ CommandCost CmdConvertRoad(TileIndex tile, DoCommandFlag flags, uint32 p1, uint3
 			}
 		}
 
+		/* Disallow converting town roads to types which do not allow houses */
+		if (rtt == RTT_ROAD && owner == OWNER_TOWN && HasBit(GetRoadTypeInfo(to_type)->flags, ROTF_NO_HOUSES)) {
+			error.MakeError(STR_ERROR_OWNED_BY);
+			GetNameOfOwner(OWNER_TOWN, tile);
+			continue;
+		}
+
 		/* Vehicle on the tile when not converting normal <-> powered
 		 * Tunnels and bridges have special check later */
 		if (tt != MP_TUNNELBRIDGE) {