Log Message:
libsixel: update to 1.8.4. (security update)

Upstream changes:

v1.8.4
* Security fix for CVE-2019-11024 (#85), recursive loop problem,
  reported by @Loginsoft-Research.

* Security fix for #73, illegal memory access problem,
  reported by @HongxuChen.

* Security fix for #89, core dumped issue,
  reported by @niugx.

* Security fix for #107, large memory allocation problem,
  reported by @cuanduo.

* Security fix for #114, heap-buffer-overflow problem,
  reported by @SuhwanSong.

* Security fix for #116, heap-buffer-overflow problem,
  reported by @SuhwanSong.

* Security fix for #118, heap-buffer-overflow problem,
  reported by @SuhwanSong.

* Security fix for #121, heap-buffer-overflow problem,
  reported by @gutiniao