kzip: tolerate an empty units directory entry (#155)
kzip: tolerate an empty units directory entry (#155)

If the zip archive contains a root/units/ entry to represent the base
directory, ensure the Scan function doesn't attempt to read the file, as it is
not a meaningful unit. Such directories are never created by this library, but
may be inserted by other generators.
bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,bzz/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,kythe/kythe,bzz/kythe,bzz/kythe,bzz/kythe,kythe/kythe,kythe/kythe