bluestore/bluestore_types: avoid heap-buffer-overflow in another way to keep code uniformity

This commit modifies end iterator to keep code uniformity as items container did.

Ref: https://github.com/ceph/ceph/pull/56890/files/19104a6fffe24a6debfd8492c90b3027126eb352#r1567083822

Fixes: https://tracker.ceph.com/issues/65482

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