Avoid a crash if subjects is empty.

This can happen in the rare instance that the last
actor in the selection is killed in the same tick
that the OG is activated, and GetCursor is called
before the next tick cancels the OG.

OpenRA.Mods.Common/Traits/AttackMove.cs

@@ -175,7 +175,7 @@ namespace OpenRA.Mods.Common.Traits
 		{
 			var prefix = mi.Modifiers.HasModifier(Modifiers.Ctrl) ? "assaultmove" : "attackmove";
 
-			if (world.Map.Contains(cell))
+			if (world.Map.Contains(cell) && subjects.Any())
 			{
 				var explored = subjects.First().Actor.Owner.Shroud.IsExplored(cell);
 				var blocked = !explored && subjects.Any(a => !a.Trait.Info.MoveIntoShroud);