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

d0f/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools