[input-output-hk/cardano-sl] [CBR-227] Avoid overflow in `updateUTxO` (#3443)

This is unlikely to happen in reality, but the tests were hitting this edge
case.

## Description

The tests were failing in https://github.com/input-output-hk/cardano-sl/pull/3439 but the failure was completely unrelated to that particular ticket.

## Linked issue

https://iohk.myjetbrains.com/youtrack/issue/CBR-227

## Type of change

— [ ] 🐞 Bug fix (non-breaking change which fixes an issue)
— [ ] 🛠 New feature (non-breaking change which adds functionality)
— [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
— [ ] 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
— [ ] 🔨 New or improved tests for existing code
— [ ] ⛑ git-flow chore (backport, hotfix, etc)

## Developer checklist

— [ ] I have read the [style guide](https://github.com/input-output-hk/cardano-sl/blob/develop/docs/style-guide.md) document, and my code follows the code style of this project.
— [ ] If my code deals with exceptions, it follows the [guidelines](https://github.com/input-output-hk/cardano-sl/blob/develop/docs/exceptions.md).
— [ ] I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on [cardanodocs.com](https://github.com/input-output-hk/cardanodocs.com), amending the inline [Haddock](https://www.haskell.org/haddock/) comments, any relevant README file or one of the document listed in the [docs](https://github.com/input-output-hk/cardano-sl/tree/develop/docs) directory.
— [ ] CHANGELOG entry has been added and is linked to the correct PR on GitHub.

## Testing checklist

— [ ] I have added tests to cover my changes.
— [ ] All new and existing tests passed.

## QA Steps

## Screenshots (if available)

You can view, comment on, or merge this pull request online at:

https://github.com/input-output-hk/cardano-sl/pull/3443

— Commit Summary —

* [CBR-227] Avoid overflow in `updateUTxO`

— File Changes —

M wallet-new/src/Cardano/Wallet/Kernel/DB/Spec.hs (3)
M wallet-new/src/Cardano/Wallet/Kernel/DB/Spec/Read.hs (12)
M wallet-new/src/Cardano/Wallet/Kernel/DB/Spec/Update.hs (7)
M wallet-new/src/Cardano/Wallet/Kernel/Transactions.hs (7)
M wallet-new/src/Cardano/Wallet/Kernel/Util/Core.hs (21)
M wallet-new/test/unit/Test/Spec/CoinSelection.hs (10)
M wallet-new/test/unit/Test/Spec/CoinSelection/Generators.hs (11)

— Patch Links —

https://github.com/input-output-hk/cardano-sl/pull/3443.patch
https://github.com/input-output-hk/cardano-sl/pull/3443.diff