Fix crash when guiding unexpectedly restarting with no current job.

Summary: When Scheduler stops activity, and specifically stops guiding, guiding timer is still active and will crash KStars when elapsing.

Test Plan:

• Make Scheduler finish a job that is guiding, so that there is no current job.
• Without the change, KStars crashes after the guider timeout elapses (hardcoded 5s).
• With the change, Scheduler does not automatically restart guiding.

Reviewers: mutlaqja, wreissenberger, alexcherney

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D24107