MFC r196962: Fix /usr/bin/unzip: A bug deep in libarchive's read-ahead logic

MFC r196962: Fix /usr/bin/unzip: A bug deep in libarchive's read-ahead logic
(incorrect handling of zero-length reads before the copy buffer is
allocated) is masked by the iso9660 taster.  Tar and cpio both enable
that taster so were protected from the bug; unzip is susceptible.

This both fixes the bug and updates the test harness to exercise
this case.

Submitted by: Ed Schouten diagnosed the bug and drafted a patch
Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@197266 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f