Fixes https://github.com/rfjakob/gocryptfs/issues/171
Steps to reproduce:
* Create a regular forward mount point
* Create a new directory in the mount point
* Manually delete the gocryptfs.diriv file from the corresponding ciphertext
directory
* Attempt to delete the directory with 'rmdir <dirname>'
Although the code explicitly checks for empty directories, it will still attempt
to move the non-existent gocryptfs.diriv file and fails with:
rmdir: failed to remove '<dirname>': No such file or directory
95870e841e