BUG/MEDIUM: peers: Handle mux creation failure.

If the mux fails to properly be created by conn_install_mux, fail, instead
of silently ignoring it.

This should be backported to 1.9.
diff --git a/src/peers.c b/src/peers.c
index bca56aa..30cf942 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -2314,7 +2314,8 @@
 	memcpy(&conn->addr.to, &peer->addr, sizeof(conn->addr.to));
 
 	conn_prepare(conn, peer->proto, peer_xprt(peer));
-	conn_install_mux(conn, &mux_pt_ops, cs, s->be, NULL);
+	if (conn_install_mux(conn, &mux_pt_ops, cs, s->be, NULL) < 0)
+		goto out_free_cs;
 	si_attach_cs(&s->si[1], cs);
 
 	s->do_log = NULL;
@@ -2328,6 +2329,8 @@
 	return appctx;
 
 	/* Error unrolling */
+out_free_cs:
+	cs_free(cs);
  out_free_conn:
 	conn_free(conn);
  out_free_strm: