Make aircraft not waste their missiles by firing from outside the map

Fixes #6001.

This obviously affects Yaks as well as Migs, even though Yaks had no
trouble with attacking from outside the map.

OpenRA.Mods.RA/Air/AttackPlane.cs

@@ -29,8 +29,8 @@ namespace OpenRA.Mods.RA.Air
 
 		protected override bool CanAttack(Actor self, Target target)
 		{
-			// dont fire while landed
-			return base.CanAttack(self, target) && self.CenterPosition.Z > 0;
+			// dont fire while landed or when outside the map
+			return base.CanAttack(self, target) && self.CenterPosition.Z > 0 && self.World.Map.Contains(self.Location);
 		}
 	}
 }