random(4): Correct a bare zero to the appropriate enum

[cem]

Summary

The convention for updating hc_destination[] is to index with a
random_entropy_source. Zero happens to match RANDOM_CACHED, which is
correct for this source (early random data). Spell the zero value as the
enum name instead of the magic constant.

No functional change.

[delphij]

Looks reasonable.

sys/dev/random/random_harvestq.c

421

@markm -- Why is this explicit_bzero() done here in the loop instead of done below (e.g. line 423)?

[markm]

sys/dev/random/random_harvestq.c

421

Over-paranoia, I guess. I wouldn't object to it moving.

[cem]

sys/dev/random/random_harvestq.c

421

One other caveat is that random_harvestq_fast_process_event -> random_fortuna_process_event() also explicit_bzeros the event in the loop already.