Fix clean_temp_cwd(): free resource allocated by opendir(3).

  *** CID 1198150:  Resource leak  (RESOURCE_LEAK)
  45        dir = opendir(".");

  [...]

  78                      /* Recurse.  */
  79                      status = clean_temp_cwd();
  80                      if (status < 0)

  >>>     CID 1198150:  Resource leak  (RESOURCE_LEAK)
  >>>     Variable "dir" going out of scope leaks the storage it points to.

  81                              return status;
  [...]

  >>>     CID 1198150:  Resource leak  (RESOURCE_LEAK)
  >>>     Variable "dir" going out of scope leaks the storage it points to.

  106             return nb_errors;
  107     }