Fix BUG 374894 - KDevelop crashes when clicking on QuickOpen

Summary:
The problem was in uninitialized value returned by ResultCache::cachedResult() which is called from ProjectItemDataProvider::itemCount() and ProjectItemDataProvider::unfilteredItemCount() methods.

The crash happens only if uninitialized value is sufficiently big and ResultCache::markDirty() was not called before.

BUG: 374894

Test Plan: Tested on master branch

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4598