Core/GameObject: Cast Pickup spell of GAMEOBJECT_TYPE_FLAGSTAND

* and do interact condition checks

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

@@ -3161,9 +3161,16 @@ void GameObject::Use(Unit* user)
                 if (player->GetVehicle())
                     return;
 
+                if (HasFlag(GO_FLAG_IN_USE))
+                    return;
+
+                if (!MeetsInteractCondition(player))
+                    return;
+
                 player->RemoveAurasByType(SPELL_AURA_MOD_STEALTH);
                 player->RemoveAurasByType(SPELL_AURA_MOD_INVISIBILITY);
-                return;                                     //we don;t need to delete flag ... it is despawned!
+                spellId = GetGOInfo()->flagStand.pickupSpell;
+                spellCaster = nullptr;
             }
             break;
         }