channel: `build_closing_transaction` i64 -> u64 underflow

[phlip9]

Hey, was taking a look at channel close fees today and noticed something funky in Channel::build_closing_transaction.

When value_to_holder: i64 or value_to_counterparty: i64 are negative (funder doesn't have enough channel balance to pay the total_fee_satoshis) they'll underflow at the _ as u64 conversion.

Adding some logs, you can see:

// context
[lightning/src/ln/channel.rs:4004:3] holder_balance = 2500
[lightning/src/ln/channel.rs:4004:3] counterparty_balance = 17500

// first block
[lightning/src/ln/channel.rs:4004:3] total_fee_satoshis = 3053
[lightning/src/ln/channel.rs:4004:3] value_to_holder = -553
[lightning/src/ln/channel.rs:4004:3] value_to_counterparty = 17500

// after if blocks
[lightning/src/ln/channel.rs:4022:3] total_fee_satoshis = 3606
[lightning/src/ln/channel.rs:4022:3] value_to_holder as u64 = 18446744073709551063
[lightning/src/ln/channel.rs:4022:3] value_to_counterparty as u64 = 17500

Fortunately the remote just force closes, so it doesn't seem too problematic.

[TheBlueMatt]

Good point, yea we should debug_assert on that not underflowing and if it does in prod just force-close the channel. I don't believe that underflow is actually reachable.

[TheBlueMatt]

Fixed in #3452.