crypto: clear err stack after ECDH::BufferToPoint

Functions that call `ECDH::BufferToPoint` were not clearing the
error stack on failure, so an invalid key could leave leftover
error state and cause subsequent (unrelated) signing operations
to fail.

PR-URL: https://github.com/nodejs/node/pull/13275


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>