crypto: use a const SSL_CIPHER

SSL_CIPHER objects are conceptually const in any case and this allows
STACK_OF(SSL_CIPHER) to return a const pointer, as is done in BoringSSL
and, perhaps, OpenSSL in the future.

PR-URL: https://github.com/nodejs/node/pull/4913


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>