mining: fix data race

[dajohi]

Use the read template.

Fixes #1893

[dnldd]

whoops, wanted to request a change there. How about using the read template after the lock:

// curTplHasNumVotes returns whether or not the current template is valid,
// builds on the provided hash, and contains the specified number of votes.
func (g *BgBlkTmplGenerator) curTplHasNumVotes(votedOnHash *chainhash.Hash, numVotes uint16) bool {
	g.templateMtx.Lock()

	template, err := g.template, g.templateErr
	g.templateMtx.Unlock()
	if template == nil || err != nil {
		return false
	}
	if template.Block.Header.PrevBlock != *votedOnHash {
		return false
	}
	return template.Block.Header.Voters == numVotes
}

[davecgh]

I agree with @dnldd here. The issue isn't that the mutex is being unlocked too early, rather that it's supposed to be using the local after the unlock.