[ruby/openssl] ssl: use SSL_get_rbio() to check if SSL is started or not

Use SSL_get_rbio() instead of SSL_get_fd(). SSL_get_fd() internally
calls SSL_get_rbio() and it's enough for our purpose.

In OpenSSL 3.0, SSL_get_fd() leaves an entry in the OpenSSL error queue
if BIO has not been set up yet, and we would have to clean it up.

https://github.com/ruby/openssl/commit/e95ee24867

ext/openssl/ossl_ssl.c

@@ -1443,8 +1443,8 @@ ossl_sslctx_flush_sessions(int argc, VALUE *argv, VALUE self)
 static inline int
 ssl_started(SSL *ssl)
 {
-    /* the FD is set in ossl_ssl_setup(), called by #connect or #accept */
-    return SSL_get_fd(ssl) >= 0;
+    /* BIO is created through ossl_ssl_setup(), called by #connect or #accept */
+    return SSL_get_rbio(ssl) != NULL;
 }
 
 static void