Fix a class of crashes in the city list

[lmoureaux]

Modifying a city can (somehow) change the selection, upon which selected_cities is modified. Modifying a QList invalidates iterators, thus we were triggering undefined behavior when doing this in loops. Copy the list before doing anything with it, which avoids any invalidation (since we work on the copy).

Also remove a duplicated nullptr check (items in the list shouldn't be nullptr anyway).

Test plan:

• Select all cities
• Switch to a governor that works for some cities but not all
• Observe crash without patch (YMMV), no crash with patch

[jwrober]

Works for me. I was able to recreate the crash and see it go away with the patch.