get_crosslink_committees reads previous_seed during genesis_epoch

[paulhauner]

Problem

Summary

During GENESIS_EPOCH, get_crosslink_committees_at_slot(..) reads state.previous_shuffling_epoch when epoch == current_epoch.

Detail

get_crosslink_committees_at_slot(...) first checks for epoch == previous_epoch, then checks for epoch == current_epoch.

During the genesis epoch, current_epoch == previous_epoch and the function so uses state.previous_shuffling_epoch == ZERO_HASH for shuffling.

During the next epoch (GENESIS_EPOCH + 1), when you wish to get the shuffling for GENESIS_EPOCH, you also trigger the current_epoch == previous_epoch path. However, state.previous_shuffling_epoch != ZERO_HASH anymore -- it is now the value that was set during get_genesis_beacon_state(...) (a.k.a. current_shuffling_seed during GENESIS_EPOCH).

Proposed Solutions

1. (Preferred) Swap the if statements for so we first check for epoch == current_epoch, then epoch == previous_epoch.
2. (Seems wrong) During get_genesis_beacon_state(..) ensure state.previous_shuffling_seed is also set to generate_seed(state, GENESIS_EPOCH).