crypto: clear error on return in TLS methods

Methods like `X509_STORE_add_cert` may push errors onto OpenSSL's error
stack. Ensure that they won't pop up in a different places like
`tls_wrap.cc`.

Fix: https://github.com/iojs/io.js/issues/712
PR-URL: https://github.com/iojs/io.js/pull/719


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>