cmd/vet: fix up some nits in print.go found by kamil.kisiel@gmail.com
cmd/vet: fix up some nits in print.go found by kamil.kisiel@gmail.com

R=golang-dev, kamil.kisiel, bradfitz
CC=golang-dev
https://codereview.appspot.com/7369049

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