o Minor bugfixes (file handling):
    - Stop failing when key files are zero-length. Instead, generate new
      keys, and overwrite the empty key files.
      Fixes bug 13111. Patch by "teor".
    - Stop generating a fresh .old RSA key file when the .old file is missing.
    - Avoid overwriting .old key files with empty key files.
    - Stop crashing when a NULL filename is passed to file_status().
      Fixed as part of bug 13111. Patches by "teor".

  o Minor enhancements (file handling):
    - Skip loading zero-length extra info store, router store, stats, state,
      and key files.
    - Return FN_ERROR when a zero-length filename is passed to file_status().
      Fixed as part of bug 13111. Patches by "teor".

  o Minor enhancements (testing):
    - Test that tor does not fail when key files are zero-length.
      Check that tor generates new keys, and overwrites the empty key files.
    - Test that tor generates new keys when keys are missing (existing
      behaviour).
    - Test that tor does not overwrite key files that already contain data
      (existing behaviour).
      Tests bug 13111. Patch by "teor".