DiffieHellman doesn't generate keys after setting a key

DiffieHellman doesn't generate keys after setting a key ## Steps To Reproduce: 1. Instantiate: `const dh = crypto.createDiffieHellman(1024);` 2. Set private key: ``` //set private key to 2 dh.setPrivateKey(Buffer.from("02", 'hex')); //outputs 02 (as expected) console.log(dh.getPrivateKey().toString('hex')); ``` 3. Generate random private key: ``` //generate random private key dh.generateKeys(); //outputs 02: zero day. console.log(dh.getPrivateKey().toString('hex')); ``` ## Underlying issue: OpenSSL (https://github.com/majek/openssl/blob/master/crypto/dh/dh_key.c) doesn't generate keys when they're already instantiated: ``` if (dh->priv_key == NULL) { priv_key=BN_new(); if (priv_key == NULL) goto err; generate_new_key=1; } else priv_key=dh->priv_key; if (dh->pub_key == NULL) { pub_key=BN_new(); if (pub_key == NULL) goto err; } else pub_key=dh->pub_key; ``` node:crypto should use OpenSSL correctly. Method `generateKeys()` should re-instantiate OpenSSL before requesting a key, thereby avoiding the above. ## Impact DiffieHellman may be used as the basis for application level security, implications are consequently broad. E.g., key reuse can cause major problems, cryptanalysis may break confidentiality, integrity, ...