Display register data payload crash fix

[soerenbf]

Purpose

Fixes an issue causing the wallet to crash when trying to display a CBOR encoded object passed to register data transaction

cc @rbaconcordium

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.