Fix setting MayYieldReservation when there is no reservation

OpenRA.Mods.Common/Activities/Air/AllowYieldingReservation.cs

@@ -0,0 +1,32 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2016 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using OpenRA.Activities;
+using OpenRA.Mods.Common.Traits;
+
+namespace OpenRA.Mods.Common.Activities
+{
+	public class AllowYieldingReservation : Activity
+	{
+		readonly Aircraft aircraft;
+
+		public AllowYieldingReservation(Actor self)
+		{
+			aircraft = self.Trait<Aircraft>();
+		}
+
+		public override Activity Tick(Actor self)
+		{
+			aircraft.AllowYieldingReservation();
+			return NextActivity;
+		}
+	}
+}