godoc: fix crash in -analysis
godoc: fix crash in -analysis

Fixes golang/go#20641

Change-Id: I499b4b8da2899c4b606ce3eedd30caab57607a85
Reviewed-on: https://go-review.googlesource.com/45570
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools