x/gov/simulation: NewDecodeStore unmarshals wrong proposal due to typo in value source

[odeke-em]

Summary of Bug

Noticed while auditing, see this code

cosmos-sdk/x/gov/simulation/decoder.go

Lines 19 to 25 in ab24d0e

	var proposalA types.Proposal
	err := cdc.UnmarshalBinaryBare(kvA.Value, &proposalA)
	if err != nil {
	panic(err)
	}
	var proposalB types.Proposal
	err = cdc.UnmarshalBinaryBare(kvA.Value, &proposalB)

notice that both ProposalA and ProposalB unmarshal from kvA.Value?
shouldn’t that use kvB.Value? Looks like a typo from PR #6147 from May 2020.

We shall also need to improve the tests to catch such bugs, given that that code has been in for 9 months without any tests catching it, shows we need to tighten the test.

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