deref_tag: handle tag->tagged = NULL

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tag.c

@@ -9,7 +9,10 @@ const char *tag_type = "tag";
 struct object *deref_tag(struct object *o, const char *warn, int warnlen)
 {
 	while (o && o->type == OBJ_TAG)
-		o = parse_object(((struct tag *)o)->tagged->sha1);
+		if (((struct tag *)o)->tagged)
+			o = parse_object(((struct tag *)o)->tagged->sha1);
+		else
+			o = NULL;
 	if (!o && warn) {
 		if (!warnlen)
 			warnlen = strlen(warn);