Fix segfault on exit.

It seems that the audio callback will keep being called after exit
unless rb->pcm_play_stop() is called again.

apps/plugins/varvara/varvara.c

@@ -411,6 +411,8 @@ enum plugin_status plugin_start(const void* parameter)
 	/* empty    */ uxn_port(&u, 0xf, nil_dei, nil_deo);
     uxn_eval(&u, 0x0100);
 	run();
+	rb->pcm_play_stop();
+	rb->pcm_set_frequency(HW_SAMPR_DEFAULT);
 	file_cleanup();
 	return PLUGIN_OK;