go/pointer: fix panic due to bogus constraint from value,ok receive operation.

See regression test for explanation.

I audited the code for similar issues and found none.

Many thanks to Daniel Morsing for providing a small
reproducible test case, a rarity for PTA bugs!

Fixes golang/go#9002

LGTM=crawshaw
R=crawshaw
CC=daniel.morsing, golang-codereviews
https://golang.org/cl/163350043