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
regexp/syntax: fix validity testing of zero repeats

This is already tested by TestRE2Exhaustive, but the build has
not broken because that test is not run when using -test.short.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/155580043