(yarrow256_fast_reseed): Set ctx->seeded = 1, so

that it is set if and only if the aes context has been initialized
with aes_set_encrypt_key.
(yarrow256_seed): No need to set ctx->seeded here.
(yarrow256_update): Likewise.

Rev: nettle/ChangeLog:1.28
Rev: nettle/yarrow256.c:1.5