Always run pending finalizers when event_base_free() is called

Always run pending finalizers when event_base_free() is called

There was actually a bug in the original version of this: it tried to
run the finalizers after (potentially) setting current_base to NULL;
but those finalizers could themselves (potentially) be invoking stuff
that needed to know about the current event_base.  So the right time to
do it is _before_ clearing current_base.