crypto: clear error stack in ECDH::Initialize

Clean up OpenSSL error stack in `ECDH::Initialize`, some curves have
faulty implementations that are leaving dangling errors after
initializing the curve.

Fix: #4686
PR-URL: https://github.com/nodejs/node/pull/4689


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>