BUG/MEDIUM: servers: Make assign_tproxy_address work when ALPN is set.

If an ALPN is set on the server line, then when we reach assign_tproxy_address,
the stream_interface's endpoint will be a connection, not a conn_stream,
so make sure assign_tproxy_address() handles both cases.

This should be backported to 1.9.
diff --git a/src/backend.c b/src/backend.c
index ab5a629..3e32797 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1021,7 +1021,12 @@
 	struct server *srv = objt_server(s->target);
 	struct conn_src *src;
 	struct connection *cli_conn;
-	struct connection *srv_conn = cs_conn(objt_cs(s->si[1].end));
+	struct connection *srv_conn;
+
+	if (objt_cs(s->si[1].end))
+		srv_conn = cs_conn(__objt_cs(s->si[1].end));
+	else
+		srv_conn = objt_conn(s->si[1].end);
 
 	if (srv && srv->conn_src.opts & CO_SRC_BIND)
 		src = &srv->conn_src;