Fix NRE in Guard when the target actor is dead.

OpenRA.Mods.Common/Traits/Guard.cs

@@ -41,7 +41,7 @@ namespace OpenRA.Mods.Common.Traits
 		public void ResolveOrder(Actor self, Order order)
 		{
 			if (order.OrderString == "Guard")
-				GuardTarget(self, Target.FromActor(order.TargetActor), order.Queued);
+				GuardTarget(self, order.Target, order.Queued);
 		}
 
 		public void GuardTarget(Actor self, Target target, bool queued = false)
@@ -49,6 +49,9 @@ namespace OpenRA.Mods.Common.Traits
 			if (!queued)
 				self.CancelActivity();
 
+			if (target.Type != TargetType.Actor)
+				return;
+
 			self.SetTargetLine(target, Color.Yellow);
 
 			var range = target.Actor.Info.TraitInfo<GuardableInfo>().Range;