HTTP-FLV: Crash when multiple viewers. v6.0.148 (#4144)

I did some preliminary code inspection. The two playback endpoints share
the same `SrsLiveStream` instance. After the first one disconnects,
`alive_` is set to false.
```
  alive_ = true;
  err = do_serve_http(w, r);
  alive_ = false;
```

In the `SrsHttpStreamServer::http_unmount(SrsRequest* r)` function,
`stream->alive()` is already false, so `mux.unhandle` will free the
`SrsLiveStream`. This causes the other connection coroutine to return to
its execution environment after the `SrsLiveStream` instance has already
been freed.
```
    // Wait for cache and stream to stop.
    int i = 0;
    for (; i < 1024; i++) {
        if (!cache->alive() && !stream->alive()) {
            break;
        }
        srs_usleep(100 * SRS_UTIME_MILLISECONDS);
    }

    // Unmount the HTTP handler, which will free the entry. Note that we must free it after cache and
    // stream stopped for it uses it.
    mux.unhandle(entry->mount, stream.get());
```

`alive_` was changed from a `bool` to an `int` to ensure that
`mux.unhandle` is only executed after each connection's `serve_http` has
exited.

---------

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

trunk/src/app/srs_app_source.hpp

@@ -529,7 +529,7 @@ private:
     SrsRtmpFormat* format_;
 private:
     // Whether source is avaiable for publishing.
-    bool _can_publish;
+    bool can_publish_;
     // The last die time, while die means neither publishers nor players.
     srs_utime_t stream_die_at_;
     // The last idle time, while idle means no players.