Fixes Generators behaviour of current() & key() when gen is closed.
Fixes Generators behaviour of current() & key() when gen is closed.

- Returned last valid value, should (and now does) return PhpValue.Null

iolevel/peachpie-concept,iolevel/peachpie,peachpiecompiler/peachpie,iolevel/peachpie-concept,peachpiecompiler/peachpie,iolevel/peachpie-concept,iolevel/peachpie,iolevel/peachpie,peachpiecompiler/peachpie