Remove an unnecessary cast in the param to BUF_MEM_grow

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/5826)

(cherry picked from commit 21db0e1a)