…#4141)

When unpublishing, the handler callback that will stop the coroutine:

```cpp
_can_publish = true;
handler->on_unpublish(req);
```

In this handler, the `http_unmount` will be called:

```cpp
void SrsHttpStreamServer::http_unmount(SrsRequest* r)
    cache->stop();
```

In this `http_unmount` function, there could be context switching. In
such a situation, a new connection might publish the stream while the
unpublish process is freeing the stream, leading to a crash.

To prevent a new publisher, we should change the state only after all
handlers and hooks are completed.

---------

Co-authored-by: liumengte <liumengte@visionular.com>
Co-authored-by: winlin <winlinvip@gmail.com>