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)