fix: reactor test failure

[egonspace]

After #56, reactor_test often failed.
For example, TestReactorValidatorSetChanges test does not finish occasionally.

The reason is that when some peers receive proposal without starting the consensus, the thread was terminated by the nil pointer during the next processing.

func (cs *State) defaultSetProposal(proposal *types.Proposal) error {
...
	// Verify signature
	if !cs.Proposer.PubKey.VerifyBytes(...) { // nil pointer referencing
		...
	}
...

}

We should not use cs.Proposer in this function.

Closes: #XXX

Description

---

For contributor use:

• Wrote tests
• Updated CHANGELOG_PENDING.md
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments
• Re-reviewed Files changed in the Github PR explorer

[zemyblue]

LGTM

[torao]

LGTM