message: Fix compilation warning when checking for writable directory
  Summary:
  As title. Fixes code introduced in previous diff.
  Fixes
diff: namespace {
  -bool check_directory(std::string& apkdir) {
  - if (!boost::filesystem::is_directory(apkdir.c_str())) {
  - std::cerr << "error: apkdir is not a writable directory: " << apkdir
  +void check_directory(std::string& dir) {
  + if (!boost::filesystem::is_directory(dir.c_str())) {
  + std::cerr << "error: not a writable directory: " << dir
  << std::endl;
  exit(EXIT_FAILURE);
  }
  @@ -22,11 +22,8 @@ FILE* ApkManager::new_asset_file(const char* filename) {
  check_directory(m_apk_dir);
  std::ostringstream path;
  path << m_apk_dir << "/assets/";
  - if (!boost::filesystem::is_directory(path.str().c_str())) {
  - std::cerr << "error: assets dir is not a writable directory: " << path.str()
  - << std::endl;
  - exit(EXIT_FAILURE);
  - }
  + std::string assets_dir = path.str();
  + check_directory(assets_dir);
  path << filename;
  FILE* fd = fopen(path.str().c_str(), "w");
  