Better way of performing the entry name sanitisation

diffoscope (76) UNRELEASED; urgency=medium

  * Don't write to arbitrary locations when extracting archive members.

    (Closes: #XXXXXX)

    (Closes: #854723)

 -- Ximin Luo <infinity0@debian.org>  Thu, 09 Feb 2017 22:14:46 +0100

                if entry.isdir:

                    continue

                clean_name = os.path.basename(entry.pathname.rstrip('/' + os.sep))

                if not clean_name:

                # All extracted locations must be underneath self._unpacked

                force_prefix = os.path.join(self._unpacked, "")

                # Try to pick a safe and reasonable candidate name

                candidate_name = os.path.normpath(entry.pathname.rstrip('/' + os.sep))

                if os.path.isabs(candidate_name):

                    candidate_name = os.path.relpath(candidate_name, os.path.join(os.path.sep))

                dst = os.path.normpath(os.path.join(self._unpacked, candidate_name))

                if not dst.startswith(force_prefix):

                    logger.warn("Skipping member because we could not make a safe name to extract it to: '%s'",

                                entry.pathname)

                    continue

                # TODO: need to fix reading these cleaned members. currently

                # reading will still try to use the uncleaned name.

                dst = os.path.join(self._unpacked, clean_name)

                #logging.debug("Extracting %s to %s", entry.pathname, dst)

                os.makedirs(os.path.dirname(dst), exist_ok=True)

                with open(dst, 'wb') as f: