tag: factor out get_tagged_oid()

Add a function for accessing the ID of the object referenced by a tag
safely, i.e. without causing a segfault when encountering a broken tag
where ->tagged is NULL.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tag.c

@@ -212,3 +212,10 @@ int parse_tag(struct tag *item)
 	free(data);
 	return ret;
 }
+
+struct object_id *get_tagged_oid(struct tag *tag)
+{
+	if (!tag->tagged)
+		die("bad tag");
+	return &tag->tagged->oid;
+}