(svn r14582) -Fix [FS#2392]: blank box on cost estimation of levelling a flat area.

-Fix: make levelling, raising and lowering of an area behave the same.

src/terraform_cmd.cpp

@@ -359,6 +359,7 @@ CommandCost CmdLevelLand(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 
 	/* Check range of destination height */
 	if (h > MAX_TILE_HEIGHT) return_cmd_error((oldh == 0) ? STR_1003_ALREADY_AT_SEA_LEVEL : STR_1004_TOO_HIGH);
+	if (p2 == 0) _error_message = STR_ALREADY_LEVELLED;
 
 	/* make sure sx,sy are smaller than ex,ey */
 	int ex = TileX(tile);