Placing spice outside the map bounds causes a crash

[abcdefg30]

Reported on Discord by Inlines:

OpenRA engine version release-20210321
Dune 2000 mod version release-20210321
on map 31ff17b01c0ded70f6f70f651d6c436241f9ec84 (The Spice Must Flow by Inlines).
Date: 2021-09-06 01:54:58Z
Operating System: Windows (Microsoft Windows NT 6.1.7601 Service Pack 1)
Runtime Version: .NET CLR 4.0.30319.42000
Exception of type `System.IndexOutOfRangeException`: Index was outside the bounds of the array.
   at OpenRA.Mods.D2k.Traits.D2kResourceRenderer.UpdateRenderedSprite(CPos cell, RendererCellContents content)
   at OpenRA.Mods.Common.Traits.ResourceRenderer.OpenRA.Traits.IWorldLoaded.WorldLoaded(World w, WorldRenderer wr)
   at OpenRA.World.LoadComplete(WorldRenderer wr)
   at OpenRA.Game.StartGame(String mapUID, WorldType type)
   at OpenRA.Mods.Common.Widgets.Logic.MainMenuLogic.<>c__DisplayClass15_0.<LoadMapIntoEditor>b__0()
   at System.Action`1.Invoke(T obj)
   at OpenRA.Game.LogicTick()
   at OpenRA.Game.Loop()
   at OpenRA.Game.Run()
   at OpenRA.Game.InitializeAndRun(String[] args)
   at OpenRA.WindowsLauncher.WindowsLauncher.RunGame(String[] args)

[pchote]

OpenRA/OpenRA.Mods.D2k/Traits/World/D2kResourceRenderer.cs

Lines 139 to 144 in e0e2197

	var directions = CVec.Directions;
	for (var i = 0; i < directions.Length; i++)
	{
	var neighbour = cell + directions[i];
	UpdateRenderedSpriteInner(neighbour, RenderContents[neighbour]);
	}

is not checking for neighbours outside the map bounds, so crashes if a cell adjacent to the map edge contains spice.