Bugfix:  There might not be a single folder.

  I'd always assumed there would be a valid list of folders beneath
 the maildir_prefix.  Mostly because what's the point of running a
 maildclient, or trying a new one, if you have no mail?!

 Anyway if ~/Maildir, or whatever valid `maildir_prefix` is configured
 to point to a directory which is empty we'd segfault on trying to open
 or toggle a folder.

  Fix this by returning if the maildir.size is zero.

  This is not a complete fix because the maildir mode should draw
 a string of the form:  "no valid maildirs found".

  Closes #9.