cmd/guru: fix nil deref when position is not in Go source

+ test

Fixes issue golang/go#14684

Change-Id: I56023bf36c307d02c71b4ddf08aee9c229fc66c1
Reviewed-on: https://go-review.googlesource.com/20247
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
3 files changed