Avoid broadcast lifecycle races in active sessions API

src/stream.cpp

@@ -1932,9 +1932,17 @@ namespace stream {
     audio::capture(session->mail, session->config.audio, session);
   }
 
+  namespace session {
+    extern std::atomic_uint running_sessions;
+  }
+
   nlohmann::json get_active_sessions_info() {
     auto result = nlohmann::json::array();
 
+    if (session::running_sessions.load(std::memory_order_relaxed) == 0) {
+      return result;
+    }
+
     auto ref = broadcast.ref();
     if (!ref) {
       return result;