修复关闭mp4录制不能触发无人观看事件问题：#2010

src/Common/MultiMediaSourceMuxer.cpp

@@ -200,6 +200,12 @@ int MultiMediaSourceMuxer::totalReaderCount(MediaSource &sender) {
 
 //此函数可能跨线程调用
 bool MultiMediaSourceMuxer::setupRecord(MediaSource &sender, Recorder::type type, bool start, const string &custom_path, size_t max_second) {
+    onceToken token(nullptr, [&]() {
+        if (_option.mp4_as_player && type == Recorder::type_mp4) {
+            //开启关闭mp4录制，触发观看人数变化相关事件
+            onReaderChanged(sender, totalReaderCount());
+        }
+    });
     switch (type) {
         case Recorder::type_hls : {
             if (start && !_hls) {