fix nftsByOwner getter about rmrk-substrate HOT 5 CLOSED

#[pallet::storage]
/// The assets held by any given account; set out this way so that assets owned by a single
/// account can be enumerated.
pub(super) type Account<T: Config<I>, I: 'static = ()> = StorageNMap<
	_,
	(
		NMapKey<Blake2_128Concat, T::AccountId>, // owner
		NMapKey<Blake2_128Concat, T::ClassId>,
		NMapKey<Blake2_128Concat, T::InstanceId>,
	),
	(),
	OptionQuery,
>;

Account::<T, I>::remove((&details.owner, &class, &instance));
Account::<T, I>::insert((&dest, &class, &instance), ());

/// Returns an iterator of the asset instances of all classes owned by `who`.
///
/// NOTE: iterating this list invokes a storage read per item.
fn owned(who: &T::AccountId) -> Box<dyn Iterator<Item = (Self::ClassId, Self::InstanceId)>> {
	Box::new(Account::<T, I>::iter_key_prefix((who,)))
}

/// Returns an iterator of the asset instances of `class` owned by `who`.
///
/// NOTE: iterating this list invokes a storage read per item.
fn owned_in_class(
	class: &Self::ClassId,
	who: &T::AccountId,
) -> Box<dyn Iterator<Item = Self::InstanceId>> {
	Box::new(Account::<T, I>::iter_key_prefix((who, class)))
}