新增媒体流flush机制：#1996

src/Record/MP4Recorder.cpp

@@ -33,6 +33,7 @@ MP4Recorder::MP4Recorder(const string &path, const string &vhost, const string &
 }
 
 MP4Recorder::~MP4Recorder() {
+    flush();
     closeFile();
 }
 
@@ -96,6 +97,12 @@ void MP4Recorder::closeFile() {
     }
 }
 
+void MP4Recorder::flush() {
+    if (_muxer) {
+        _muxer->flush();
+    }
+}
+
 bool MP4Recorder::inputFrame(const Frame::Ptr &frame) {
     if (!(_have_video && frame->getTrackType() == TrackAudio)) {
         //如果有视频且输入的是音频，那么应该忽略切片逻辑