Test tx lookup validity in CheckInputTransactions.

This commit modifies CheckInputTransactions to ensure that not only must a
transaction exist in the transaction store, it must also not have any
errors associated with it or be nil.

validate.go

@@ -604,7 +604,7 @@ func CheckTransactionInputs(tx *btcwire.MsgTx, txHeight int64, txStore TxStore)
 		// Ensure the input is available.
 		txInHash := &txIn.PreviousOutpoint.Hash
 		originTx, exists := txStore[*txInHash]
-		if !exists {
+		if !exists || originTx.Err != nil || originTx.Tx == nil {
 			str := fmt.Sprintf("unable to find input transaction "+
 				"%v for transaction %v", txInHash, txHash)
 			return 0, RuleError(str)