Core/Combat: Properly unset engagement flag on death.

Port From (https://github.com/TrinityCore/TrinityCore/commit/6b1ca0ca128425677043d36a147a019f6d2ac6e6)

Source/Game/AI/CoreAI/CreatureAI.cs

@@ -244,6 +244,12 @@ namespace Game.AI
             if (!IsEngaged())
                 return false;
 
+            if (!me.IsAlive())
+            {
+                EngagementOver();
+                return false;
+            }
+
             if (!me.HasReactState(ReactStates.Passive))
             {
                 Unit victim = me.SelectVictim();

Source/Game/Entities/Creature/Creature.cs

@@ -502,6 +502,9 @@ namespace Game.Entities
                     if (m_deathState != DeathState.Corpse)
                         break;
 
+                    if (IsEngaged())
+                        AIUpdateTick(diff);
+
                     if (m_groupLootTimer != 0 && !lootingGroupLowGUID.IsEmpty())
                     {
                         if (m_groupLootTimer <= diff)
@@ -583,7 +586,6 @@ namespace Game.Entities
                         }
                     }
 
-                    // do not allow the AI to be changed during update
                     AIUpdateTick(diff);
 
                     if (!IsAlive())
@@ -1087,7 +1089,7 @@ namespace Game.Entities
             base.AtDisengage();
 
             ClearUnitState(UnitState.AttackPlayer);
-            if (HasDynamicFlag(UnitDynFlags.Tapped))
+            if (IsAlive() && HasDynamicFlag(UnitDynFlags.Tapped))
                 SetDynamicFlags((UnitDynFlags)GetCreatureTemplate().DynamicFlags);
 
             if (IsPet() || IsGuardian()) // update pets' speed for catchup OOC speed

Source/Game/Entities/Unit/Unit.Combat.cs

@@ -858,8 +858,6 @@ namespace Game.Entities
 
                 if (!creature.IsPet())
                 {
-                    creature.GetThreatManager().ClearAllThreat();
-
                     // must be after setDeathState which resets dynamic flags
                     if (!creature.loot.IsLooted())
                         creature.AddDynamicFlag(UnitDynFlags.Lootable);