staking/api/commission: fix possible panic in validation check

[ptrus]

The validation check would panic whenever the number of bound steps was greater than rate_steps + 2.

[Yawning]

Add an error case test?

[ptrus]

Add an error case test?

Not sure wym. The panic happened regardless if the bounds were vaild or not. The updated test does test this (e.g. it panics on old code & no error in new code).

Also the specific error is already tested here: https://github.com/oasislabs/oasis-core/blob/f2dee245b174ac0384908010a498e0fe9a9650a3/go/staking/api/commission_test.go#L521

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Edit: i guess it would be nicer if i add a specific test case for this with an explicit comment on what it is testing. Done.

[codecov]

Codecov Report

Merging #2763 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2763      +/-   ##
==========================================
- Coverage   62.52%   62.41%   -0.12%     
==========================================
  Files         385      385              
  Lines       36406    36406              
==========================================
- Hits        22763    22721      -42     
- Misses      10748    10818      +70     
+ Partials     2895     2867      -28

Impacted Files	Coverage Δ
go/staking/api/commission.go	98.66% <100%> (ø)	⬆️
go/worker/compute/executor/committee/state.go	74.07% <0%> (-11.12%)	⬇️
go/oasis-node/cmd/debug/byzantine/byzantine.go	35.01% <0%> (-5.04%)	⬇️
go/consensus/api/grpc.go	59.3% <0%> (-4.66%)	⬇️
go/worker/common/host/protocol/protocol.go	65.67% <0%> (-4.48%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	60.18% <0%> (-4.02%)	⬇️
go/worker/common/p2p/p2p.go	67.56% <0%> (-3.61%)	⬇️
go/worker/compute/executor/committee/node.go	62% <0%> (-2.96%)	⬇️
go/oasis-node/cmd/debug/byzantine/merge.go	74.68% <0%> (-2.54%)	⬇️
go/runtime/client/client.go	66.28% <0%> (-2.3%)	⬇️
... and 17 more

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