Fix all wandering actors wandering at first game tick

NotifyBecomingIdle is only notified at the first idle tick, so `countdown` is not set when TickIdle checks it at first game tick. Therefore, we let TickIdle return early at first game tick to avoid that.

OpenRA.Mods.Common/Traits/Wanders.cs

@@ -39,6 +39,7 @@ namespace OpenRA.Mods.Common.Traits
 		int countdown;
 		int ticksIdle;
 		int effectiveMoveRadius;
+		bool firstTick = true;
 
 		public Wanders(Actor self, WandersInfo info)
 		{
@@ -54,6 +55,13 @@ namespace OpenRA.Mods.Common.Traits
 
 		public void TickIdle(Actor self)
 		{
+			// The countdown has not have been set at this point, so don't check yet
+			if (firstTick)
+			{
+				firstTick = false;
+				return;
+			}
+
 			if (--countdown > 0)
 				return;