lib9: fix warning under clang 3.1
lib9: fix warning under clang 3.1

Fixes issue 3534.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6035054

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang