[release-branch.go1] os/exec: don't crash when out of fds

««« backport 00e1feaa9218
os/exec: don't crash when out of fds

Command.Start could crash before if no fds were available
because a nil *os.File of /dev/null was added to the cleanup
list, which crashed before returning the proper error.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6514043

»»»