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>

pack-bitmap.c

@@ -709,9 +709,7 @@ struct bitmap_index *prepare_bitmap_walk(struct rev_info *revs)
 			else
 				object_list_insert(object, &wants);
 
-			if (!tag->tagged)
-				die("bad tag");
-			object = parse_object_or_die(&tag->tagged->oid, NULL);
+			object = parse_object_or_die(get_tagged_oid(tag), NULL);
 		}
 
 		if (object->flags & UNINTERESTING)

revision.c

@@ -404,9 +404,7 @@ static struct commit *handle_commit(struct rev_info *revs,
 		struct tag *tag = (struct tag *) object;
 		if (revs->tag_objects && !(flags & UNINTERESTING))
 			add_pending_object(revs, object, tag->tag);
-		if (!tag->tagged)
-			die("bad tag");
-		object = parse_object(revs->repo, &tag->tagged->oid);
+		object = parse_object(revs->repo, get_tagged_oid(tag));
 		if (!object) {
 			if (revs->ignore_missing_links || (flags & UNINTERESTING))
 				return NULL;

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;
+}

tag.h

@@ -19,5 +19,6 @@ struct object *deref_tag(struct repository *r, struct object *, const char *, in
 struct object *deref_tag_noverify(struct object *);
 int gpg_verify_tag(const struct object_id *oid,
 		   const char *name_to_report, unsigned flags);
+struct object_id *get_tagged_oid(struct tag *tag);
 
 #endif /* TAG_H */