Log Message:
htmldoc: update to 1.9.13.

# Changes in HTMLDOC v1.9.13

- Now install a 32x32 icon for Linux (Issue #432)
- Fixed an issue with large values for roman numerals and letters in headings
  (Issue #433)
- Fixed a crash bug when a HTML comment contains an invalid nul character
  (Issue #439)
- Fixed a crash bug with bogus BMP images (Issue #444)
- Fixed a potential heap overflow bug with bogus GIF images (Issue #451)
- Fixed a potential stack overflow bug with bogus BMP images (Issue #453)