Some hash functions in the freebl3 library don't support truncated
digests and either return immediately with no update of the output
length, or ignore the length of the output buffer and always write whole
digest.
Initialize the return value to zero to get correct result with the
former.
This is triggered only in the hash unit test. chronyd always provides a
sufficient buffer for the digest.
This should reduce the number of possible memory leaks reported by
valgrind. The remaining reported leaks are sched tqe allocation, async
DNS instance allocation, cmdmon response/timestamp cell allocation, and
clientlog subnet allocation.
