Log Message:
libsixel: update to 1.8.5. (security update)

Upstream changes (from NEWS):

v1.8.5

* Security fix for CVE-2019-20205 (#127), integer overflow problem,
  reported by @sleicasper.

* Security fix for CVE-2019-20056 (#126), assertion failure problem,
  reported by @sleicasper.

* Security fix for CVE-2019-20094 (#125), heap overflow problem,
  reported by @cuanduo.

* Security fix for #124, illegal longjump() call problem,
  reported by @cuanduo.

* Serucity fix for #74 and #123, access violation problem,
  reported by @HongxuChen and SuhwanSong.

* Security fix for #122, heap overflow problem,
  reported by @SuhwanSong.

* Security fix for CVE-2019-20023(#117, #119, #120), memory leaks problem,
  reported by @SuhwanSong and @gutiniao.

* Strip first flag check in LZW compression function for issue #118,
  reported by @yoichi