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