Log Message:
libsixel: update to 1.8.3. (security fixes)

Upstream changes:

v1.8.3
* Security fix for CVE-2018-19757 (#79), NULL pointer dereference problem,
  reported by @nluedtke and fixed by @knok (#91, #94).

* Security fix for CVE-2018-19762 (#81), heap-based buffer overflow problem,
  reported by @nluedtke and fixed by @knok (#92).

* Security fix for CVE-2018-19756 (#80), heap-based buffer over-read problem,
  reported by @nluedtke and fixed by @knok (#93).

* Security fix for CVE-2018-19763 (#82), heap-based buffer over-read problem,
  reported by @nluedtke and fixed by @knok (#95).

* Security fix for CVE-2018-19761, illegal address access, fixed by @knok (#96).

* Security fix for CVE-2018-19759, heap-based buffer over-read problem, fixed by \ 
@knok (#98).

* Security fix for CVE-2018-3753 (#83), infinite loop problem,
  reported by @cool-tomato and fixed by @knok (#99).

* Security fix for CVE-2018-19759 (#102),
  heap-based buffer over-read that will cause a denial of service.
  reported and fixed by @YourButterfly. (#106)

* Security fix for CVE-2019-19635 (#103), heap-based buffer overflow,
  reported and fixed by @YourButterfly. (#106)

* Security fix for CVE-2019-19636 (#104) and CVE-2019-19637 (#105), integer \ 
overflow problem.
  reported and fixed by @YourButterfly. (#106)

* gif loader: check LZW code size (Issue #75), Thanks to @HongxuChen.
              \ 
https://github.com/saitoha/libsixel/commit/7808a06b88c11dbc502318cdd51fa374f8cd47ee

* core: Fix a global-buffer-overflow problem (Issue #72), Thanks to @fgeek.
        \ 
https://github.com/saitoha/libsixel/commit/c868b59ec89bdb24c42a0de89e5319a989076c66

* core: Fix unexpected hangs/performance issues (Issue #76), Thanks to @HongxuChen.
        \ 
https://github.com/saitoha/libsixel/commit/88561b7a810017b91d26b6273323dde4b6f9b273
        \ 
https://github.com/saitoha/libsixel/commit/2d3d9ffe8ab886b7bc670fd896d63c628436cc66
        \ 
https://github.com/saitoha/libsixel/commit/c9363cd1d5929e1d721af9f09633061dfa8152fe