runtime: zero d.free field
runtime: zero d.free field

Not programming in Go anymore:
have to clear fields in local variables.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7002053

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang