MINOR: stream: link the stream to its session

Now each stream is added to the session's list of streams, so that it
will be possible to know all the streams belonging to a session, and
to know if any stream is still attached to a sessoin.
diff --git a/src/peers.c b/src/peers.c
index 7bf7766..249edf7 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1856,6 +1856,7 @@
 
 	/* Error unrolling */
  out_free_strm:
+	LIST_DEL(&s->by_sess);
 	LIST_DEL(&s->list);
 	pool_free2(pool2_stream, s);
  out_free_task: