Fix potential arithmetic overflow during highway rewards calculation

[RitaMAllenCA]

There is a potential for arithmetic overflow at https://github.com/casper-network/casper-node/blob/f7d8228de3cb56a3fe705f5a787d3db[…]/components/consensus/highway_core/finality_detector/rewards.rs if the quorum weight is u64::MAX. This can happen if e.g. we have two stakes of au64::MAX and bu64::MAX where each stake is divisible by a+b. Examples of pairs for a and b are (514, 771) or (668, 614).