Core/GameObects: Fix a crash in GameObject::Delete

src/server/game/Entities/GameObject/GameObject.cpp

@@ -610,6 +610,8 @@ void GameObject::Delete()
     if (GetOwnerGUID())
         if (Unit* owner = GetOwner())
             owner->RemoveGameObject(this, false);
+        else    //! Owner not in world anymore
+            SetOwnerGUID(0);
 
     ASSERT (!GetOwnerGUID());
     SendObjectDeSpawnAnim(GetGUID());