kbfscrypto: clean up bytes access while making hash key
kbfscrypto: clean up bytes access while making hash key

Suggested by jzila.

Issue: #1836

keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client