cmd/dist: fix crash on windows currently 'dist install' always crashes on windows
cmd/dist: fix crash on windows
currently 'dist install' always crashes on windows

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/11919043

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