Fix faulty identity selector

[shjortConcordium]

Purpose

Fix that sometimes the account credential issuer / identityProviderIndex was incorrect.

Changes

Fix lookup of identity, which just chose the first identity always.

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.