guard: ensure guard->path is always freed
Normally 'path' gets freed in bolt_guard_remove, which is called from the dispose method of the BoltGuard. There is one exception though: when the pipe (named fifo) is still active, we do not unlink the path, since we want to recover that guard later. In that case the memory for path will be leaked. Add an additional g_clear_pointer for path in the 'finalize' method to ensure that path always gets freed.