Make minimum size for secure memory a size_t.

Make minimum size for secure memory a size_t.

The minimum size argument to CRYPTO_secure_malloc_init() was an int but ought
to be a size_t since it is a size.

From an API perspective, this is a change.  However, the minimum size is
verified as being a positive power of two and it will typically be a small
constant.

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from #11003)