cmd/dist: fix build for Linux/ARM         find() returns -1 when not found.
cmd/dist: fix build for Linux/ARM
        find() returns -1 when not found.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5752068

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