containments/desktop: Execute updatePositionsList only if we know how many icons per stripe
updatePositionsList() is not always guaranteed to be called after setPerStripe has been called. setPerStripe is called by rowsInserted and setPerStripe by the QML ui so if the model is ready before the ui we'll have a divide by zero
(cherry picked from commit 239ffd70)
239ffd70 containments/desktop: Execute updatePositionsList only if we know how many icons per stripe
Co-authored-by: Marco Martin notmart@gmail.com