deref_tag: handle return value NULL

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

builtin-show-ref.c

@@ -86,6 +86,9 @@ static int show_ref(const char *refname, const unsigned char *sha1, int flag, vo
 			    sha1_to_hex(sha1));
 		if (obj->type == OBJ_TAG) {
 			obj = deref_tag(obj, refname, 0);
+			if (!obj)
+				die("git-show-ref: bad tag at ref %s (%s)", refname,
+				    sha1_to_hex(sha1));
 			hex = find_unique_abbrev(obj->sha1, abbrev);
 			printf("%s %s^{}\n", hex, refname);
 		}

merge-recursive.c

@@ -1673,6 +1673,8 @@ static struct commit *get_ref(const char *ref)
 	if (get_sha1(ref, sha1))
 		die("Could not resolve ref '%s'", ref);
 	object = deref_tag(parse_object(sha1), ref, strlen(ref));
+	if (!object)
+		return NULL;
 	if (object->type == OBJ_TREE)
 		return make_virtual_commit((struct tree*)object,
 			better_branch_name(ref));

sha1_name.c

@@ -578,8 +578,11 @@ static int handle_one_ref(const char *path,
 	struct object *object = parse_object(sha1);
 	if (!object)
 		return 0;
-	if (object->type == OBJ_TAG)
+	if (object->type == OBJ_TAG) {
 		object = deref_tag(object, path, strlen(path));
+		if (!object)
+			return 0;
+	}
 	if (object->type != OBJ_COMMIT)
 		return 0;
 	insert_by_date((struct commit *)object, list);

shallow.c

@@ -56,7 +56,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 			if (i < heads->nr) {
 				commit = (struct commit *)
 					deref_tag(heads->objects[i++].item, NULL, 0);
-				if (commit->object.type != OBJ_COMMIT) {
+				if (!commit || commit->object.type != OBJ_COMMIT) {
 					commit = NULL;
 					continue;
 				}

upload-pack.c

@@ -575,7 +575,8 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 	}
 	if (o->type == OBJ_TAG) {
 		o = deref_tag(o, refname, 0);
-		packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname);
+		if (o)
+			packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname);
 	}
 	return 0;
 }