core/types: handle nil ChainId in NewEIP155Signer

All uses of ChainConfig.ChainId eventually end up in NewEIP155Signer.
This fixes the case where users forget to set the ChainId in their
config.

core/types/transaction_signing.go

@@ -112,6 +112,9 @@ type EIP155Signer struct {
 }
 
 func NewEIP155Signer(chainId *big.Int) EIP155Signer {
+	if chainId == nil {
+		chainId = new(big.Int)
+	}
 	return EIP155Signer{
 		chainId:    chainId,
 		chainIdMul: new(big.Int).Mul(chainId, big.NewInt(2)),