reflect: declare slice as *[]unsafe.Pointer instead of *[]byte

reflect: declare slice as *[]unsafe.Pointer instead of *[]byte

The new garbage collector (CL 6114046) may find the fake *[]byte value
and interpret its contents as bytes rather than as potential pointers.
This may lead the garbage collector to free memory blocks that
shouldn't be freed.

R=dvyukov, rsc, dave, minux.ma, remyoudompheng, iant
CC=golang-dev
https://golang.org/cl/7000059