NULL terminate the built in "help" argv array to avoid
reading beyond the end.
Reviewed-by: Richard Levitte <levitte@openssl.org> Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com> Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/12258)