Log Message:
htmldoc: update to 1.9.15.

# Changes in HTMLDOC v1.9.15

- Fixed a potential heap overflow bug with GIF images (Issue #461)
- Fixed a potential double-free bug with PNG images (Issue #462)
- Fixed a potential stack overflow bug with GIF images (Issue #463)
- Fixed a potential heap underflow bug with empty attributes (Issue #464)
- Fixed a potential stack overflow bug with BMP images (Issue #466)
- Fixed a potential heap overflow bug with the table-of-contents (Issue #467)
- Fixed a potential heap overflow bug with headings (Issue #468)
- Fixed a potential stack overflow bug with GIF images (Issue #470)