Fix leaking of requests in ItemRetriever::exec()

Summary: This one is serious because it impacts actual runtime, not just tests.

Test Plan: ctest -R itemretrievertest in an ASAN build

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: anthonyfieroni, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21121