Fix regression in Windows implementation of KUniqueService

Initialize the static instance pointer when a responder was created.
Additionally, clear the static instance pointer when the instance is
destroyed and add some error handling to detect wrong usage more
quickly.

(cherry picked from commit bc1d094c04b5fa9629231f5830d9ecc8a1f00e19)

• Regression-due-to: 56784e6ba086825b2e076ac35feefac6d7055ca3
• GnuPG-bug-id: T7528