message: Fix issues when `walletdir` is root directory
  + Remove one character less from wallet path if root directory
diff: std::vector<fs::path> ListDatabases(const fs::path& wallet_dir)
  {
  - const size_t offset = wallet_dir.string().size() + 1;
  + const size_t offset = wallet_dir.string().size() + (wallet_dir == wallet_dir.root_name() ? 0 : 1);
  std::vector<fs::path> paths;
  boost::system::error_code ec;
  