godoc: if there is no search box, don't run the respective JS code
godoc: if there is no search box, don't run the respective JS code

Fixes a JS crash which prevented the automatically generated
table of contents to not appear.

R=adg, bradfitz
CC=golang-dev
http://codereview.appspot.com/4460041

dsriram/golang,ssrl/go,dsriram/golang,ssrl/go,ssrl/go,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,ssrl/go,dsriram/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,ssrl/go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang