Fix test-dump-cache-tree in one-tree disappeared case.

When reconstructing an invalidated subtree for reference purposes by
test-dump-cache-tree, we did not handle the case where we shouldn't
have a cached and invalidated subtree in the result, leading to an
unneeded die().

Signed-off-by: Junio C Hamano <junkio@cox.net>

dump-cache-tree.c

@@ -21,10 +21,9 @@ static int dump_cache_tree(struct cache_tree *it,
 	int i;
 	int errs = 0;
 
-	if (!it)
-		return;
-	if (!ref)
-		die("internal error");
+	if (!it || !ref)
+		/* missing in either */
+		return 0;
 
 	if (it->entry_count < 0) {
 		dump_one(it, pfx, "");