cmd/godoc: don't crash if there's no documentation
cmd/godoc: don't crash if there's no documentation

Fixes regression introduced by CL 7860049.

R=golang-dev, kamil.kisiel, dave
CC=golang-dev
https://codereview.appspot.com/8069044

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