OPENSSL_cleanup: cleanup secure memory

If the global DRBGs are allocated on the secure heap, then calling
CRYPTO_secure_malloc_done() inside main() will have no effect, unless
OPENSSL_cleanup() has been called explicitely before that, because
otherwise the DRBGs will still be allocated. So it is better to cleanup
the secure heap automatically at the end of OPENSSL_cleanup().

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5294)