rf: panic in Refactor.Load

commit 6fb3311e1224f1af9839bdef5161e043863fe93f

While trying to create minimal example to demonstrate #9, I found the following panic.
Reproduce by running the testscript command:

exec rf 'ex {import "example/c"; Foo -> Bar}'
-- a/a.go --
package a

import (
	"example/b"
	"example/c"
)

var F = b.F

func init() {
	b.F = new(c.Foo)
}
-- a/a_test.go --
package a_test

import (
	"example/b"
	"example/c"
	"testing"
)

func TestFoo(t *testing.T) {
	b.F = new(c.Foo)
}
-- b/b.go --
package b

import "example/c"

var F *c.Foo
-- c/c.go --
package c

type Foo int
-- go.mod --
module example

go 1.17

I see this panic:

panic executing: 
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x64e7ce]

goroutine 1 [running]:
main.run.func1(0xc000179ed0)
	/home/rogpeppe/other/rf/rf.go:76 +0xe7
panic(0x6b28e0, 0x8905e0)
	/home/rogpeppe/go/src/runtime/panic.go:972 +0x1d4
rsc.io/rf/refactor.(*Refactor).Load(0xc00005a480, 0xc000070930, 0xc000014210, 0x2)
	/home/rogpeppe/other/rf/refactor/snap.go:336 +0x19ce
main.run(0xc00005a480, 0x7ffff469abc9, 0x23, 0x0, 0x0)
	/home/rogpeppe/other/rf/rf.go:101 +0x238
main.main()
	/home/rogpeppe/other/rf/rf.go:44 +0x1bb