vim: fix CVE-2021-3796, CVE-2021-3872, and CVE-2021-3875

Backport patches from upstream to fix these CVEs.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b493eb4f)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>