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
https://golang.org/cl/6488106

src/cmd/godoc/format.go

@@ -108,7 +108,7 @@ func FormatSelections(w io.Writer, text []byte, lw LinkWriter, links Selection,
 			break
 		}
 		// determine the kind of segment change
-		if index == len(selections)-1 {
+		if lw != nil && index == len(selections)-1 {
 			// we have a link segment change:
 			// format the previous selection segment, write the
 			// link tag and start a new selection segment