crypto: check for OpenSSL errors when signing

Errors might be injected into OpenSSL's error stack
without the return value of `PEM_read_bio_PrivateKey` being set to
`nullptr`. See the test of `test_bad_rsa_privkey.pem` for an
example.

PR-URL: https://github.com/nodejs/node/pull/2342


Reviewed-By: Fedor Indutny <fedor@indutny.com>