fix segfault when ASYNC thread is disabled

lib/src/phy/rf/rf_uhd_imp.c

@@ -615,8 +615,11 @@ int rf_uhd_close(void *h)
   uhd_tx_metadata_free(&handler->tx_md);
   uhd_rx_metadata_free(&handler->rx_md_first);
   uhd_rx_metadata_free(&handler->rx_md);
+
+#if HAVE_ASYNC_THREAD
   handler->async_thread_running = false;
   pthread_join(handler->async_thread, NULL);
+#endif
 
   uhd_tx_streamer_free(&handler->tx_stream);
   uhd_rx_streamer_free(&handler->rx_stream);