lib9: fix memory leak in p9getwd()         although the comment says it uses libc's getenv, without NOPLAN9DEFINES         it actually uses p9getenv which strdups.
lib9: fix memory leak in p9getwd()
        although the comment says it uses libc's getenv, without NOPLAN9DEFINES
        it actually uses p9getenv which strdups.

R=golang-dev, dave, rsc
CC=golang-dev
http://codereview.appspot.com/6285046

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