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.
              saitoha/libsixel@7808a06

* core: Fix a global-buffer-overflow problem (Issue #72), Thanks to @fgeek.
        saitoha/libsixel@c868b59

* core: Fix unexpected hangs/performance issues (Issue #76), Thanks to @hongxuchen.
        saitoha/libsixel@88561b7
        saitoha/libsixel@2d3d9ff
        saitoha/libsixel@c9363cd

graphics/libsixel/Makefile

@@ -1,7 +1,6 @@
-# $NetBSD: Makefile,v 1.42 2019/08/22 12:23:20 ryoon Exp $
+# $NetBSD: Makefile,v 1.43 2019/12/18 14:02:10 tsutsui Exp $
 
-DISTNAME=		libsixel-1.8.2
-PKGREVISION=		6
+DISTNAME=		libsixel-1.8.3
 CATEGORIES=		graphics
 MASTER_SITES=		${MASTER_SITE_GITHUB:=saitoha/}
 GITHUB_PROJECT=		libsixel

graphics/libsixel/distinfo

@@ -1,6 +1,6 @@
-$NetBSD: distinfo,v 1.21 2018/07/28 14:01:21 tsutsui Exp $
+$NetBSD: distinfo,v 1.22 2019/12/18 14:02:10 tsutsui Exp $
 
-SHA1 (libsixel-1.8.2.tar.gz) = f0a150461904d4217617009249c63150182588f7
-RMD160 (libsixel-1.8.2.tar.gz) = 1f1a6f5a46fa36ecab2563a96c4913d96a61713e
-SHA512 (libsixel-1.8.2.tar.gz) = f0287b7b7a190f0f964e27fc65aa816a78c09de5e50045a25eb05f9b430c74f779a7d5eb591b3430a82a06621d53719bf115bcdd0b99acc9afe3cb9ab6fde271
-Size (libsixel-1.8.2.tar.gz) = 4778776 bytes
+SHA1 (libsixel-1.8.3.tar.gz) = 5c1360b56b1259fca4ac1055da1fbbd29cd16d6b
+RMD160 (libsixel-1.8.3.tar.gz) = 9874ba7263e1add4079aeea73dfb6e63bea09309
+SHA512 (libsixel-1.8.3.tar.gz) = 1b62083cbbaf7c8d05de646aa854c9e472f65ab9e34c6fdccbb2c283da4fdcb9838eddba41ea6a9720315e735bbf97cf6446dc6cb46fbf73d4b173068d18fda8
+Size (libsixel-1.8.3.tar.gz) = 4781583 bytes