Core/Movement: Add some extra assertions to MotionMaster.

Port From (https://github.com/TrinityCore/TrinityCore/commit/792914fb518349ef270e6f769f343bbcd333b942)

Source/Game/Handlers/TaxiHandler.cs

@@ -130,8 +130,6 @@ namespace Game
             if (GetPlayer().HasUnitState(UnitState.Died))
                 GetPlayer().RemoveAurasByType(AuraType.FeignDeath);
 
-            GetPlayer().GetMotionMaster().Remove(MovementGeneratorType.Flight);
-
             if (mountDisplayId != 0)
                 GetPlayer().Mount(mountDisplayId);
 

Source/Game/Movement/Generators/FlightPathMovementGenerator.cs

@@ -234,7 +234,12 @@ namespace Game.Movement
         {
             int nodeCount = _path.Count;        //! Number of nodes in path.
             _endMapId = _path[nodeCount - 1].ContinentID; //! MapId of last node
-            _preloadTargetNode = (uint)nodeCount - 3;
+
+            if (nodeCount < 3)
+                _preloadTargetNode = 0;
+            else
+                _preloadTargetNode = (uint)nodeCount - 3;
+
             _endGridX = _path[nodeCount - 1].Loc.X;
             _endGridY = _path[nodeCount - 1].Loc.Y;
         }

Source/Game/Movement/MotionMaster.cs

@@ -884,7 +884,8 @@ namespace Game.Movement
                     Log.outDebug(LogFilter.Server, $"MotionMaster::MoveTaxiFlight: {_owner.GetGUID()} taxi to Path Id: {path} (node {pathnode})");
 
                     // Only one FLIGHT_MOTION_TYPE is allowed
-                    Remove(MovementGeneratorType.Flight);
+                    bool hasExisting = HasMovementGenerator(gen => gen.GetMovementGeneratorType() == MovementGeneratorType.Flight);
+                    Cypher.Assert(!hasExisting, "Duplicate flight path movement generator");
 
                     FlightPathMovementGenerator movement = new();
                     movement.LoadPath(_owner.ToPlayer());