Fix atomSetCache not protected
Fix atomSetCache not protected

Clones atomSet in structure.atomSetCache so as to prevent cache
corruption by returning reference to the cached atomSet

j0kaso/ngl,arose/ngl,arose/ngl,fredludlow/ngl,fredludlow/ngl,j0kaso/ngl,j0kaso/ngl,arose/ngl,fredludlow/ngl,arose/ngl,fredludlow/ngl