Integrate perlio:

[ 15274]
Other :encoding oddities
- we should really pop while we can still make method calls
in case DESTROY does something.
- Provide a no-op DESTROY
- Use SvPV_only() rather than just SvPV_on()

[ 15273]
Avoid overwrite of un-allocated memory when SvCUR > SvLEN
p4raw-link: @15274 on //depot/perlio: 284ee456826016ea93485015648c2b3b4b3bcd40
p4raw-link: @15273 on //depot/perlio: 54f0641bc82f8965b371b5750b4424f8aef9918d

p4raw-id: //depot/perl@15275