give all pointers in slavebase a default init to nullptr

Summary:
lest they don't get initialized and then end up crashing should they get
deleted (as is the case right now with configGroup)

Test Plan: no longer segfaulting every time rebuildConfig is run during ctest

Reviewers: dfaure, meven, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24655