ppp: fix CVE-2022-4603

<CVE-2022-4603>
Avoid out-of-range access to packet buffer
Upstream-Status: Backport[https://github.com/ppp-project/ppp/commit/a75fb7b198eed50d769c80c36629f38346882cbf

]

Signed-off-by: Minjae Kim <flowergom@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>