Fix the TxIn index of the return collateral.

[sjoerdvisscher]

Pre-submit checklist:

• Branch
  • [ ] Tests are provided (if possible)
  • [ ] Commit sequence broadly makes sense and have useful messages
  • [ ] Important changes are reflected in changelog.d of the affected packages
  • [ ] Relevant tickets are mentioned in commit messages
• PR
  • [ ] (For external contributions) Corresponding issue exists and is linked in the description
  • [ ] Targeting main unless this is a cherry-pick backport
  • [ ] Self-reviewed the diff
  • [ ] Useful pull request description
  • [ ] If relevant, reference the ADR in the PR and reference the PR in the ADR
  • [ ] Reviewer requested