godoc: fix crash in -analysis

Fixes golang/go#20641

Change-Id: I499b4b8da2899c4b606ce3eedd30caab57607a85
Reviewed-on: https://go-review.googlesource.com/45570
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed