Core/Creatures: Remove stand state when a creature attacks or dies

Port From (https://github.com/TrinityCore/TrinityCore/commit/c05466fa0c2dc9b07528ed967d77bf0313fdafff)

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

@@ -354,8 +354,9 @@ namespace Game.Entities
                 creature.SendAIReaction(AiReaction.Hostile);
                 creature.CallAssistance();
 
-                // Remove emote state - will be restored on creature reset
+                // Remove emote and stand state - will be restored on creature reset
                 SetEmoteState(Emote.OneshotNone);
+                SetStandState(UnitStandStateType.Stand);
             }
 
             // delay offhand weapon attack by 50% of the base attack time

Source/Game/Entities/Unit/Unit.cs

@@ -1551,6 +1551,7 @@ namespace Game.Entities
                 SetHealth(0);
                 SetPower(GetPowerType(), 0);
                 SetEmoteState(Emote.OneshotNone);
+                SetStandState(UnitStandStateType.Stand);
 
                 // players in instance don't have ZoneScript, but they have InstanceScript
                 ZoneScript zoneScript = GetZoneScript() != null ? GetZoneScript() : GetInstanceScript();