fix forking while iterating over file handles with events, specifically using $_

There are probably two bugs here:

1. Using the topic `$_` when iterating over a list with the possibility of forking
2. autovivifying entries in `$reactor_poll->{io}{fileno $f}` by requesting it's `{cb}` value

The first is the issue causing a windows core dump, but I think the second might prevent stopping the loop correctly on the next iteration, when in this iteration `$reactor_poll->reset` is called