Fix possible crash in KStandardItemListWidget::paint()

According to the backtrace in the bug report, it is possible that
KStandardItemListWidget::paint() is called if the hash m_textInfo has
not been initialised. The widget's index must be -1 in this case, see
KStandardItemListWidget::triggerCacheRefreshing(). It looks like this
can only happen if the item is about to be removed from the view, see
KItemListView::slotItemsRemoved().

I could not reproduce the crash, so I'm not sure why exactly this
happens, but this commit should at least prevent the crash.

BUG: 306167
FIXED-IN: 4.9.2