Fixes to GrepOutputView toolbar

Summary:
Fixes to GrepOutputView toolbar

• fix crash when aborting grep jobs at startup by clicking on "Clear Search History" (introduced in commit 42eb6bb62c28)
• correctly disable result navigation buttons (fixes crash when clicking on "Collapse All" button for an empty result)

Test Plan:

• quit KDevelop with a long GrepJob in history
• restart KDevelop and abort job by clicking on "Clear Search History" -> no crash

• start a long GrepJob and stop it by clicking on "Clear Search History"
• navigation buttons are now disabled

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6542