Monitor#mon_exit

static VALUE
monitor_exit(VALUE monitor)
{
    monitor_check_owner(monitor);

    struct rb_monitor *mc = monitor_ptr(monitor);

    if (mc->count <= 0) rb_bug("monitor_exit: count:%d\n", (int)mc->count);
    mc->count--;

    if (mc->count == 0) {
        RB_OBJ_WRITE(monitor, &mc->owner, Qnil);
        rb_mutex_unlock(mc->mutex);
    }
    return Qnil;
}