Log Message:
libsixel: update to 1.8.2.

Upstream changes:
 (from https://github.com/saitoha/libsixel/releases/tag/v1.8.2 )

v1.8.2

This release provides some security updates.

* core: Fix memory leak problems(#67, CVE-2018-14072, CVE-2018-14073),
  thanks to @fCorleone.
  f94bc6f
  84ed0bc
* core: Fix some heap buffer-overflow problems(#68, #69, #70, #71),
  thanks to @fgeek.
  6a19d99
  0d70e04
  438188c
  ba21bb9
* man: Fix a typo (#66), thanks to @tsutsui.
  cf47281