fs-util: Add missing assert to chmod_and_chown_at()

src/basic/fs-util.c

@@ -197,6 +197,8 @@ int readlink_and_make_absolute(const char *p, char **r) {
 int chmod_and_chown_at(int dir_fd, const char *path, mode_t mode, uid_t uid, gid_t gid) {
         _cleanup_close_ int fd = -1;
 
+        assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
+
         if (path) {
                 /* Let's acquire an O_PATH fd, as precaution to change mode/owner on the same file */
                 fd = openat(dir_fd, path, O_PATH|O_CLOEXEC|O_NOFOLLOW);