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

bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh