修复forward崩溃

@@ -68,8 +68,6 @@ SrsForwarder::SrsForwarder(SrsSource* _source)
 
 SrsForwarder::~SrsForwarder()
 {
-    on_unpublish();
-    
     srs_freep(pthread);
     srs_freep(queue);
     srs_freep(jitter);

@@ -237,6 +237,7 @@ void srs_close_stfd(st_netfd_t& stfd)
         // we must ensure the close is ok.
         int err = st_netfd_close(stfd);
         srs_assert(err != -1);
+        stfd = NULL;
     }
 }