Fix _addClient to avoid locking in user callback

Author: mathieucarbou

src/AsyncEventSource.cpp

@@ -368,13 +368,16 @@ void AsyncEventSource::_addClient(AsyncEventSourceClient *client) {
   if (!client) {
     return;
   }
+
+  if (_connectcb) {
+    _connectcb(client);
+  }
+
 #ifdef ESP32
   std::lock_guard<std::recursive_mutex> lock(_client_queue_lock);
 #endif
+
   _clients.emplace_back(client);
-  if (_connectcb) {
-    _connectcb(client);
-  }
 
   _adjust_inflight_window();
 }