Core/Unit: Fix units on vehicles not dismounting on death

Port From (https://github.com/TrinityCore/TrinityCore/commit/b45277f1cf52483f0471f4aa79ca6b288180ee7c)

Source/Game/Entities/Unit/Unit.cs

@@ -1363,6 +1363,8 @@ namespace Game.Entities
             // Death state needs to be updated before RemoveAllAurasOnDeath() is called, to prevent entering combat
             m_deathState = s;
 
+            bool isOnVehicle = GetVehicle() != null;
+
             if (s != DeathState.Alive && s != DeathState.JustRespawned)
             {
                 CombatStop();
@@ -1383,6 +1385,10 @@ namespace Game.Entities
                 // remove aurastates allowing special moves
                 ClearAllReactives();
                 m_Diminishing.Clear();
+
+                // Don't clear the movement if the Unit was on a vehicle as we are exiting now
+                if (!isOnVehicle)
+                {
                     if (IsInWorld)
                     {
                         // Only clear MotionMaster for entities that exists in world
@@ -1395,6 +1401,8 @@ namespace Game.Entities
                     }
                     StopMoving();
                     DisableSpline();
+                }
+
                 // without this when removing IncreaseMaxHealth aura player may stuck with 1 hp
                 // do not why since in IncreaseMaxHealth currenthealth is checked
                 SetHealth(0);