crypto/mem.c: on Windows, use rand() instead of random()

Windows doesn't provide random().  In this particular case, our
requirements on the quality of randomness isn't high, so we don't
need to care how good randomness rand() does or doesn't provide.

Fixes #3778

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3779)