Use constexpr to check memo validity

[a-noni-mousse]

High Level Overview of Change

Context of Change

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 not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release

[a-noni-mousse]

Thanks I have used your fix into my code

[intelliot]

1. You could add tests to this pull request or
2. I could submit a pull request that adds tests for Memo.

I've written such tests, and your code passes them. You can see those tests here: https://github.com/scottschurr/rippled/commits/memo-test

You're welcome to add those tests (or similar) to this pull request. Or we can leave this pull request as it is and I'll submit a separate pull request that adds the tests to the code base. Either way the code base eventually has the same outcome.

For posterity: these tests were added by https://github.com/XRPLF/rippled/pull/4287