[core] Set closing state for a broken link

Co-authored-by: Mikołaj Małecki <ethouris@gmail.com>

srtcore/core.cpp

@@ -11276,6 +11276,7 @@ void CUDT::checkTimers()
 
 void CUDT::updateBrokenConnection()
 {
+    m_bClosing = true;
     releaseSynch();
     // app can call any UDT API to learn the connection_broken error
     s_UDTUnited.m_EPoll.update_events(m_SocketID, m_sPollID, SRT_EPOLL_IN | SRT_EPOLL_OUT | SRT_EPOLL_ERR, true);