Zero memory in CRYPTO_secure_malloc.

This commit destroys the free list pointers which would otherwise be
present in the returned memory blocks.  This in turn helps prevent
information leakage from the secure memory area.

Note: CRYPTO_secure_malloc is not guaranteed to return zeroed memory:
before the secure memory system is initialised or if it isn't implemented.

[manual merge of #7011]

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7026)