Fix load_shards_from_disk/2

load_shards_from_disk/2 did not expect #ordered_shards to be returned
from load_shards_from_disk/1. Since it uses a list comprehension the
mistake is silently squashed, resulting in an empty list.

In production this manifests are the occasional failure, where 'n' is
calculated as 0, causing quorum reads to fail. It corrects itself as
calls to load_shards_from_disk/1 will return the right result and,
critically, insert the shards into mem3's cache. Subsequent /2 calls
will use the cached values until they're ousted.

BugzID: 20629