Always return NULL from fdFree()

Always return NULL from fdFree()

- Up to now, if the fd had remaining references fdFree() would return
  the supposedly free'd fd back to us, which is unlike anything else
  in rpm. Make this consistent with the rest of rpm finally as the
  last remaining caller requiring the old semantics is gone from
  the codebase (somewhere between 4.9 and 4.10): always return NULL,
  as the referenced instance is now gone as far as the caller is concerned.