KCrash: fix crash (ironic heh) when used in an app without QCoreApplication

Summary:
For instance in kio_help. The right fix is to create a QCoreApplication
in all kioslaves, but this commit preserves compatibility between KF 5.50
and 5.51 after KIO's commit d428fc8e6447ede81f1e1911d0b66b39265672f3

Test Plan: Starting khelpcenter, no more core files from kio_help in my cores directory.

Reviewers: heikobecker, aacid, sitter

Reviewed By: heikobecker

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16183