debug/pe: remove use of unsafe
debug/pe: remove use of unsafe

Helps in environments with restricted support for unsafe.

LGTM=bradfitz
R=r, bradfitz
CC=dsymonds, golang-codereviews
https://codereview.appspot.com/156410044

wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old