fdo#81516: vcl: limit number of CFFs read from font

(cherry picked from commit 45b0b47d114437198c9e0872d427576e6e7e6cc6)

Conflicts:
	vcl/source/fontsubset/cff.cxx

Change-Id: I9928b9805169a2dbb41be669dc37617b30bc672b
Reviewed-on: https://gerrit.libreoffice.org/10752
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>