If there is an error reading or parsing an archive header, the
    Archive_iterator code would return a dummy header but would not mark
    itself as done.  The effect is that an invalid archive leads to an
    endless loop reading and re-reading the same archive header.  Avoid
    that by setting the offset to the end of the file, which will cause
    the iterator to == archive_end.
    
    No test since it doesn't seem worth constructing an invalid archive.
    
    Reviewed-on: https://go-review.googlesource.com/c/158217

From-SVN: r268129