This appears to be the cause of a failure in the CSV tests as
discussed at jruby#6157.

The following code should restart the Enumerator and print "0"
twice, but the second `next` triggers a "dead fiber" error without
this patch:

```ruby
Enumerator.new {
  2.times {|i| raise i.to_s }
}.tap {|f|
  2.times { f.next rescue puts $!.message }
}
```