Correct documented return value for BIO_get_mem_data()

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9643)

(cherry picked from commit 797a5b7af9d9bbfbcbff4607c10ad5c5595ac785)