io: implement pre- and post-order directory traversal for windows

io/sys/windows/path.c

@@ -156,7 +156,9 @@ struct b_path *b_path_join(
 	}
 
 	for (size_t i = 0; i < nr_paths; i++) {
-		append_path(result, paths[i]);
+		if (paths[i]) {
+			append_path(result, paths[i]);
+		}
 	}
 
 	return result;
@@ -213,6 +215,11 @@ bool b_path_is_directory(const struct b_path *path)
 
 }
 
+const char *b_path_ptr(const struct b_path *path)
+{
+	return b_string_ptr(path->pathstr);
+}
+
 struct b_path *b_path_retain(struct b_path *path)
 {
 }