Connection: simplify uint marshaling
To prevent a common crash
This happens only for the purpose plugin, which is the only one using this code path.
This crash happened 6.7k times in the last 30 days: https://crash-reports.kde.org/organizations/kde/issues/9180/?project=11&query=is%3Aunresolved&referrer=issue-stream&statsPeriod=30d&stream_index=2