assertion in get_active_index_root too strong

[djrtwo]

Issue

In validator registry and shuffling seed data we set state.current_calculation_epoch = next_epoch and then do state.current_epoch_seed = generate_seed(state, state.current_calculation_epoch), but the assertion in get_active_index_root (used in generate_seed) asserts that the largest epoch you can access is current_epoch.

Proposed Solution

Allow access of next_epoch (current_epoch + 1). This is safe as long as we make sure we run "Set state.latest_index_roots[next_epoch % LATEST_INDEX_ROOTS_LENGTH] = hash_tree_root(get_active_validator_indices(state, next_epoch)) before any calls to get_active_index_root which we currently do.