Ensure order-of-operations for shuffling shard mod

[paulhauner]

What

Add some brackets to update_registry_and_shuffling_data to ensure shuffling_start_shard < SHARD_COUNT.

Why

The following code runs and prints "end"

epoch_committee_count = 2047;
SHARD_COUNT = 1024;
old_shuffling_start = 1023;

current_shuffling_start_shard = (
	old_shuffling_start +
	epoch_committee_count % SHARD_COUNT
)

assert(current_shuffling_start_shard > SHARD_COUNT)

current_shuffling_start_shard = (
	(old_shuffling_start +
	epoch_committee_count) % SHARD_COUNT
)

assert(current_shuffling_start_shard < SHARD_COUNT)

print("end")

[JustinDrake]

This bug fix was subsumed in #808

[djrtwo]

Actually going to express this to master for v0.5.1

[djrtwo]

closing in favor of #821