推流器主动断开时，避免开启断流续推功能

src/Rtmp/RtmpSession.cpp

@@ -219,10 +219,11 @@ void RtmpSession::onCmd_publish(AMFDecoder &dec) {
 }
 
 void RtmpSession::onCmd_deleteStream(AMFDecoder &dec) {
+    _push_src = nullptr;
+    //此时回复可能触发broken pipe事件，从而直接触发onError回调；所以需要先把_push_src置空，防止触发断流续推功能
     sendStatus({ "level", "status",
                  "code", "NetStream.Unpublish.Success",
                  "description", "Stop publishing." });
-    _push_src = nullptr;
     throw std::runtime_error(StrPrinter << "Stop publishing" << endl);
 }