godoc: fix out-of-bounds panic when serving top-level files

Change-Id: I0ba84bac0c97715c0bc66fdc4c33678341ef140c
Reviewed-on: https://go-review.googlesource.com/53151
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>