Fix event crash in mines final

Fixes http://smf.cataclysmdda.com/index.php?topic=7406

Iterator debugging helped here. It was actually the EVENT_SPAWN_WYRMS that would add a new event during `event::actualize`, which is called by `game::process_events`. The game function iterates over the events vector, adding an entry to that vector invalidates the iterator. This won't happen with `std::list`, adding entries to lists does not invalidate any of its iterators.