CipherSizeToPlainSize: Handle illegal states
CipherSizeToPlainSize: Handle illegal states

A file never gets a cipherSize <= HEADER_LEN in normal operation.
However, this can happen if header write it interrupted or the
underlying filesystem does not support fallocate.

Noticed while trying to store a CIPHERDIR in another gocryptfs mount
(gocryptfs does not support fallocate)

davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs