KIO: fix long-standing memory leak on exit.

Summary:
Slave wasn't really deleted (too late for deleteLater when
the KIO Scheduler global object is being deleted).
But actually deleting led to SlaveKeeper using dangling pointers,
so clear it first.

Detected by heaptrack.

Test Plan:
heaptrack bin/jobtest ; valgrind bin/jobtest (to check no use of deleted memory)

Before: total memory leaked 337 kB
After: total memory leaked 232 kB

Reviewers: mwolff

Reviewed By: mwolff

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7529