Fix 2-years regression in KUrlComboBox, setUrl() didn't append anymore.
This is why the top combo in the file dialog never had more than 5 items (4 default ones + the current URL).
commit f71092c8 (https://phabricator.kde.org/D14666) broke it due to switching to std::vector and using unsigned ints, turning negative numbers (for which qMax used to return 0) into very big positive numbers (thus skipping all items).