Issue 407: Tar reader tries to examine last character of an empty filename

Issue 407: Tar reader tries to examine last character of an empty filename

Of interest:  While working on this, I noted that we have
an existing test for tar files with empty filenames.
That test asserts that the correct behavior here is for the
format handler to return the entry with the empty filename
and a status of ARCHIVE_OK.  Clients need to be robust against
empty filenames.