DefaultLeaderElector::elect_leader panics for party_weights summing to 1

[matteojug]

DefaultLeaderElector::elect_leader (specifically, hash_to_range) panics for party_weights = vec![1] (or summing to 1):

#[test]
fn test_total_weight_1() {
   let mut party = MockParty::default();
   party.cfg.party_weights = vec![1];

   let elector = DefaultLeaderElector::new();
   dbg!(elector.elect_leader(&party).unwrap());
}

Note that while some of the scenarios above may not be really realistic in terms of BPCon, still given the general purpose of this it would be better to have safeguards in place to avoid runtime panics.