Security fix release

• 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, reported by @nluedtke) and CVE-2019-19778 (#110, reported by @SuhwanSong),
  heap-based buffer over-read problem, 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-2019-3753 (#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.
  7808a06

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

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