vim: fix CVE-2021-3968 and CVE-2021-3973

Backport a fix for -3972, and whitelist -3968: it isn't valid as it
fixes a bug which was introduced after 8.2.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bec5caad)
Signed-off-by: Steve Sakoman <steve@sakoman.com>