CLEANUP: backend: Move mux install to call it at only one place

It makes the code readability simpler. It will also ease futur changes.
diff --git a/src/backend.c b/src/backend.c
index b2b7ba5..f7e5dbd 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1178,22 +1178,20 @@
 		srv_conn->target = s->target;
 
 		/* set the correct protocol on the output stream interface */
-		if (srv) {
+		if (srv)
 			conn_prepare(srv_conn, protocol_by_family(srv_conn->addr.to.ss_family), srv->xprt);
-			/* XXX: Pick the right mux, when we finally have one */
-			conn_install_mux(srv_conn, &mux_pt_ops, srv_cs);
-		}
 		else if (obj_type(s->target) == OBJ_TYPE_PROXY) {
 			/* proxies exclusively run on raw_sock right now */
 			conn_prepare(srv_conn, protocol_by_family(srv_conn->addr.to.ss_family), xprt_get(XPRT_RAW));
 			if (!objt_cs(s->si[1].end) || !objt_cs(s->si[1].end)->conn->ctrl)
 				return SF_ERR_INTERNAL;
-			/* XXX: Pick the right mux, when we finally have one */
-			conn_install_mux(srv_conn, &mux_pt_ops, srv_cs);
 		}
 		else
 			return SF_ERR_INTERNAL;  /* how did we get there ? */
 
+		/* XXX: Pick the right mux, when we finally have one */
+		conn_install_mux(srv_conn, &mux_pt_ops, srv_cs);
+
 		/* process the case where the server requires the PROXY protocol to be sent */
 		srv_conn->send_proxy_ofs = 0;
 		cli_conn = objt_conn(strm_orig(s));