vim: fix CVE-2021-3927 and CVE-2021-3928

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2001631e)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>