Make sure that each time a function returns KRB5_NOMEM, everything
allocated earlier in the function and stack tree is freed.

Overwrite cache file with nulls before removing it.

Check return values and sanity-check parameters more thoroughly.  This
code was derived from UNIX file I/O code, and the conversion of
error-trapping may be incomplete.  Probably lots of bugs dealing with
end-of-file versus other errors.