go.tools/go/types: fix bad function comparison
go.tools/go/types: fix bad function comparison

Issue found by vet -nilfunc.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/12816044

t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools