Fix #7945: Add cost of clearing the sloped tile when placing a dock (#7947)

regression/regression/result.txt

@@ -7327,7 +7327,7 @@ ERROR: IsEnd() is invalid as Begin() is never called
   IsBuoyTile():         true
   IsLockTile():         true
   IsCanalTile():        true
-  GetBankBalance():     1999965070
+  GetBankBalance():     1999965040
 
 --AIWaypointList(BUOY)--
   Count():             1
@@ -7346,7 +7346,7 @@ ERROR: IsEnd() is invalid as Begin() is never called
   IsBuoyTile():         false
   IsLockTile():         false
   IsCanalTile():        false
-  GetBankBalance():     1999959675
+  GetBankBalance():     1999959645
   BuildWaterDepot():    true
   BuildDock():          true
 

src/station_cmd.cpp

@@ -2535,8 +2535,10 @@ CommandCost CmdBuildDock(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 
 	if (IsBridgeAbove(tile)) return_cmd_error(STR_ERROR_MUST_DEMOLISH_BRIDGE_FIRST);
 
+	CommandCost cost(EXPENSES_CONSTRUCTION, _price[PR_BUILD_STATION_DOCK]);
 	ret = DoCommand(tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 	if (ret.Failed()) return ret;
+	cost.AddCost(ret);
 
 	TileIndex tile_cur = tile + TileOffsByDiagDir(direction);
 
@@ -2591,7 +2593,7 @@ CommandCost CmdBuildDock(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 		st->AfterStationTileSetChange(true, STATION_DOCK);
 	}
 
-	return CommandCost(EXPENSES_CONSTRUCTION, _price[PR_BUILD_STATION_DOCK]);
+	return cost;
 }
 
 void RemoveDockingTile(TileIndex t)