cmd/godoc: fix minor bug in FormatSelections.
cmd/godoc: fix minor bug in FormatSelections.

FormatSelections tries to call a nil function value if lw is nil
and the final entry in the selections array is non-nil. Luckily,
this doesn't actually happen in practice since godoc doesn't use
this combination (no line numbers, but with selections).

R=gri
CC=gobot, golang-dev
http://codereview.appspot.com/6488106

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang