added IsDead property; made attack notice when the target is dead

OpenRa.Game/Actor.cs

@@ -79,6 +79,8 @@ namespace OpenRa.Game
 			}
 		}
 
+		public bool IsDead { get { return Health <= 0; } }
+
 		public void InflictDamage(Actor attacker, Bullet inflictor, int damage)
 		{
 			/* todo: auto-retaliate, etc */

OpenRa.Game/Traits/AttackTurreted.cs

@@ -28,6 +28,12 @@ namespace OpenRa.Game.Traits
 			if( target == null )
 				return;
 
+			if (target.IsDead)	/* stop firing on targets after we've killed them */
+			{
+				target = null;
+				return;
+			}
+
 			var turreted = self.traits.Get<Turreted>();
 			turreted.desiredFacing = Util.GetFacing( target.CenterLocation - self.CenterLocation, turreted.turretFacing );
 			if( turreted.desiredFacing != turreted.turretFacing )