Check that all of the files contained in the archive are within the
        target directory.

def check_files(self, to_path=None):
        """
        Check that all of the files contained in the archive are within the
        target directory.
        """
        if to_path:
            target_path = os.path.normpath(os.path.realpath(to_path))
        else:
            target_path = os.getcwd()
        for filename in self.filenames():
            extract_path = os.path.join(target_path, filename)
            extract_path = os.path.normpath(os.path.realpath(extract_path))
            if not extract_path.startswith(target_path):
                raise UnsafeArchive(
                    "Archive member destination is outside the target"
                    " directory.  member: %s" % filename)