Put SSL_pending() call behind the new internal SSL API.

It seems likely that any SSL implementation will need a similar call, not
just OpenSSL.

src/interfaces/libpq/fe-secure-openssl.c

@@ -186,6 +186,15 @@ pgtls_open_client(PGconn *conn)
 	return open_client_SSL(conn);
 }
 
+/*
+ *  Is there unread data waiting in the SSL read buffer?
+ */
+bool
+pgtls_read_pending(PGconn *conn)
+{
+	return SSL_pending(conn->ssl);
+}
+
 /*
  *	Read data from a secure connection.
  *