go/types: be robust in the presence of incorrect/missing position info
go/types: be robust in the presence of incorrect/missing position info

Fixes go/loader test crash.

TBR: adonovan

Change-Id: I91dba5e001afa0ee188ccea4db904a6ce744c4d0
Reviewed-on: https://go-review.googlesource.com/11042
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

syreclabs/go-tools,cw2018/tools,cmars/tools,sshetty10/tools,nicot/tools,magiccao/tools,josharian/tools,runcom/tools,glasser/golang-tools,springning/tools,sshetty10/tools,sshetty10/tools,golang/tools,nicot/tools,cmars/tools,glasser/golang-tools,matryer/tools,nicot/tools,magiccao/tools,jdhenke/tools,springning/tools,syreclabs/go-tools,sshetty10/tools,glasser/golang-tools,josharian/tools,argandas/tools,jdhenke/tools,argandas/tools,syreclabs/go-tools,springning/tools,jdhenke/tools,glasser/golang-tools,matryer/tools,matryer/tools,golang/tools,josharian/tools,golang/tools,syreclabs/go-tools,josharian/tools,argandas/tools,magiccao/tools,runcom/tools,jdhenke/tools,golang/tools,springning/tools,cw2018/tools,cmars/tools,syreclabs/go-tools,runcom/tools,runcom/tools,golang/tools,nicot/tools,magiccao/tools,cmars/tools,matryer/tools,argandas/tools,syreclabs/go-tools,cw2018/tools,cw2018/tools