Jacek Sieka 6f7652d330
process_attestation: Validate epoch before using it
`data.target.epoch` is used to count the active validator set.

Because `get_committee_count_per_slot` is extremely inefficient the way
the spec is written, clients cache it, or the underlying active
validator set.

Performing the checks in the given order leads to a (very unlikely)
security issue where the a cold and above all, distant value may get
used which may be costly - reordering the checks brings the value into a
more reasonable range before using it.
2020-07-24 07:26:37 +02:00
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