Fix: wrong error message when building canals over ship depots / locks

IsTileType() also considers ship depots and locks water. IsWaterTile() does the right thing.

src/water_cmd.cpp

@@ -475,10 +475,11 @@ CommandCost CmdBuildCanal(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 			return_cmd_error(STR_ERROR_FLAT_LAND_REQUIRED);
 		}
 
+		bool water = IsWaterTile(current_tile);
+
 		/* can't make water of water! */
-		if (IsTileType(current_tile, MP_WATER) && (!IsTileOwner(current_tile, OWNER_WATER) || wc == WATER_CLASS_SEA)) continue;
+		if (water && (!IsTileOwner(current_tile, OWNER_WATER) || wc == WATER_CLASS_SEA)) continue;
 
-		bool water = IsWaterTile(current_tile);
 		ret = DoCommand(current_tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 		if (ret.Failed()) return ret;