Core/Movement: Fix a potential crash on newly created pets.

Port From (https://github.com/TrinityCore/TrinityCore/commit/3d9d6fbcb80eaab72acbe6ef09b9b1c29f2439c7)

Source/Game/Entities/Unit/Unit.Movement.cs

@@ -523,7 +523,7 @@ namespace Game.Entities
 
                 if (creature.HasUnitTypeMask(UnitTypeMask.Minion) && !creature.IsInCombat())
                 {
-                    var top = creature.GetMotionMaster().Top();
+                    var top = creature.GetMotionMaster().TopOrNull();
                     if (top != null && top.GetMovementGeneratorType() == MovementGeneratorType.Follow)
                     {
                         Unit followed = ((AbstractFollower)top).GetTarget();

Source/Game/Movement/Generators/ChaseMovementGenerator.cs

@@ -195,7 +195,7 @@ namespace Game.Movement
 
         static bool IsMutualChase(Unit owner, Unit target)
         {
-            IMovementGenerator gen = target.GetMotionMaster().Top();
+            IMovementGenerator gen = target.GetMotionMaster().TopOrNull();
             if (gen == null || gen.GetMovementGeneratorType() != MovementGeneratorType.Chase)
                 return false;
             return ((ChaseMovementGenerator)gen).GetTarget() == owner;

Source/Game/Movement/MotionMaster.cs

@@ -804,6 +804,11 @@ namespace Game.Movement
 
         int Size() { return _top + 1; }
 
+        public IMovementGenerator TopOrNull()
+        {
+            return Empty() ? null : Top();
+        }
+        
         public IMovementGenerator Top()
         {
             Cypher.Assert(!Empty());