Legacy Litecoin address can have same prefix as Bech32

[onyb]

Opening this issue for someone working on Litecoin integration in future.

There is a 1 in 40000 chance that a legacy P2PKH Litecoin address starts with ltc1, which is normally reserved for native segwit (bech32) addresses.

Checklist:

• Figure out why this happens only with Litecoin.
  • Can this happen with other coins? What are the odds?
• Find out if btcutil might generate such an address.
• Find out if this issue has been auto-resolved in newer versions of btcutil.
  • Unlikely if this is a rare edge cas found only in Litecoin.
• Add a generic test case here, or in btcutil, to ensure it is handled properly by us.

A quick fix in one of btcutil's fork is available here: jakm/btcutil@fbb6f71.