update-index: fix a memleak

`old` is not used outside the loop and would get lost
once we reach the goto.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/update-index.c

@@ -584,6 +584,7 @@ static int do_reupdate(int ac, const char **av,
 		path = xstrdup(ce->name);
 		update_one(path);
 		free(path);
+		free(old);
 		if (save_nr != active_nr)
 			goto redo;
 	}