textproc/libxml2: Update to 2.9.9 [1]

While here, fix a bug in libxslt triggered by the libxml2 update

PR: 235713 [1]
PR: 238522
Exp-run by: antoine
Reported by: kunda <chitty_cloud@me.com> [1]
Reviewed by: kwm [1] [2]
Obtained from: https://gitlab.gnome.org/GNOME/libxslt/commit/5b0965010abf274f7a3a1291d16dde34c167e8a7