SetField doesn't appear to work

[eric-s-raymond]

I get the following trace:

`
--- FAIL: TestGetSetAttr (0.00s)
panic: reflect: call of reflect.Value.Elem on struct Value [recovered]
panic: reflect: call of reflect.Value.Elem on struct Value

goroutine 38 [running]:
testing.tRunner.func1(0xc4202544b0)
/usr/lib/go-1.10/src/testing/testing.go:742 +0x29d
panic(0x5d92e0, 0xc4201e9760)
/usr/lib/go-1.10/src/runtime/panic.go:502 +0x229
reflect.Value.Elem(0x5ef860, 0xc4202159e0, 0x99, 0xc4202159e0, 0x99, 0x20)
/usr/lib/go-1.10/src/reflect/value.go:775 +0x1b7
goreposurgeon.SetField(0x5ef860, 0xc4202159e0, 0x618dc0, 0x3, 0x5cb320, 0x64de00, 0x7, 0x61a0b5)
/home/esr/public_html/reposurgeon/src/goreposurgeon/goreposurgeon_test.go:1597 +0x6f
goreposurgeon.TestGetSetAttr(0xc4202544b0)
/home/esr/public_html/reposurgeon/src/goreposurgeon/goreposurgeon_test.go:1644 +0x1fa
testing.tRunner(0xc4202544b0, 0x632770)
/usr/lib/go-1.10/src/testing/testing.go:777 +0xd0
created by testing.(*T).Run
/usr/lib/go-1.10/src/testing/testing.go:824 +0x2e0
FAIL goreposurgeon 0.017s
`

That appears to relate to this line: structValue := reflect.ValueOf(obj).Elem()

If I remove the Elem() call, the function runs to completion but silently fails. e.g the field is not set.