blockchain: Do not accept orphans/genesis block.

This modifies the code in maybeAcceptBlock to return an error if there
is no previous entry in the block index for a given block indicating it
is either an orphan or the genesis block.

The function should never be called in either circumstance since the
genesis block is valid by definition and orphans are handled prior to
calling the function.

blockchain/accept.go

@@ -118,6 +118,14 @@ func (b *BlockChain) maybeAcceptBlock(block *dcrutil.Block, flags BehaviorFlags)
 		return false, err
 	}
 
+	// This function should never be called with orphan blocks or the
+	// genesis block.
+	if prevNode == nil {
+		prevHash := &block.MsgBlock().Header.PrevBlock
+		str := fmt.Sprintf("previous block %s is not known", prevHash)
+		return false, ruleError(ErrMissingParent, str)
+	}
+
 	blockHeight := block.Height()
 
 	// The block must pass all of the validation rules which depend on the
@@ -154,11 +162,9 @@ func (b *BlockChain) maybeAcceptBlock(block *dcrutil.Block, flags BehaviorFlags)
 	blockHeader := &block.MsgBlock().Header
 	newNode := newBlockNode(blockHeader,
 		stake.FindSpentTicketsInBlock(block.MsgBlock()))
-	if prevNode != nil {
-		newNode.parent = prevNode
-		newNode.height = blockHeight
-		newNode.workSum.Add(prevNode.workSum, newNode.workSum)
-	}
+	newNode.parent = prevNode
+	newNode.height = blockHeight
+	newNode.workSum.Add(prevNode.workSum, newNode.workSum)
 
 	// Fetching a stake node could enable a new DoS vector, so restrict
 	// this only to blocks that are recent in history.