The next segment might not be available yet when we try to
read ahead.

Should fix this assertion failure during shutdown for the most part
(draining a storage still is not guaranteed to succeed):

Assert error in LRU_Free(), storage/storage_lru.c line 77:
  Condition((((&lru->lru_head)->vtqh_first == ((void*)0))) != 0) not true.

Avoid users accidentally running into a file being created where
block devices would normally live.

Motivated by #2

I had missed to move these when I changed the locking scheme
of rewrites before the public release, and I find it curious
that my test systems did not complain.

Anyway, fixes #1