A tag object which lacks newlines won't be parsed correctly.
Git fails to detect this error and crashes due to a NULL deref:

$ git archive 1.0.0
Segmentation fault (core dumped)
$ git checkout 1.0.0
Segmentation fault (core dumped)
$

See the attached tarball for a reproduction repository.
Also mirrored at https://stsp.name/git-checkout-tag-segv-repo.tgz

With the patch below:

$ git checkout 1.0.0
fatal: reference is not a tree: 1.0.0
$ git archive 1.0.0
fatal: not a tree object: a99665eea5ee50171b5b7249880aa2ae35e35823
$

diff --git a/tree.c b/tree.c
index 4720945e6a..92d8bd57a3 100644
--- a/tree.c
+++ b/tree.c
@@ -252,9 +252,11 @@ struct tree *parse_tree_indirect(const struct object_id *oid)
 			return (struct tree *) obj;
 		else if (obj->type == OBJ_COMMIT)
 			obj = &(get_commit_tree(((struct commit *)obj))->object);
-		else if (obj->type == OBJ_TAG)
+		else if (obj->type == OBJ_TAG) {
 			obj = ((struct tag *) obj)->tagged;
-		else
+			if (!obj)
+				return NULL;
+		} else
 			return NULL;
 		if (!obj->parsed)
 			parse_object(the_repository, &obj->oid);