crypto: clear openssl error stack when handled

Clear OpenSSL's error stack on return from Connection::HandleSSLError().
This stops stale errors from popping up later in the lifecycle of the
SSL connection where they would cause spurious failures.

This commit causes a 1-2% performance regression on `make bench-tls`.
We'll address that in follow-up commits if possible but let's ensure
correctness first.

Backport of c6e2db2

src/node_crypto.cc

@@ -711,6 +711,9 @@ int Connection::HandleBIOError(BIO *bio, const char* func, int rv) {
 
 
 int Connection::HandleSSLError(const char* func, int rv) {
+  ClearErrorOnReturn clear_error_on_return;
+  (void) &clear_error_on_return;  // Silence unused variable warning.
+
   if (rv >= 0) return rv;
 
   int err = SSL_get_error(ssl_, rv);