D17017: Fix assert when cleaning up symlinks in PreviewJob

[Elvis Angelaccio]

elvisangelaccio added a reviewer: dfaure.

REPOSITORY
  R241 KIO

REVISION DETAIL
  https://phabricator.kde.org/D17017

To: davidedmundson, dfaure
Cc: kde-frameworks-devel, michaelh, ngraham, bruns