When creating a cbc encrypter/decrypter it doesn't check for nil iv (for example, I was
expecting it to use "default" iv of all zeros in that case), and causes a
confusing panic much later, when calling CryptBlocks. Post Go 1 it would likely be
better to:

- (optional) check iv for nil and use make([]byte, BlockSize) to create a
"zero" iv
- panic if iv size is less than BlockSize, so it is clear where is the actual error