ApplyPrivateMethod return interface is invalid in 1.16

[maodou1990]

PrivateMethodStruct add two private methods:

func (this *PrivateMethodStruct) lastError() error{
    return nil
}

func (this *PrivateMethodStruct) LastError() error{
    return this.lastError()
}

TestApplyPrivateMethod add test below:

Convey("patch private method return interface", func() {
	f := new(fake.PrivateMethodStruct)
	var s *fake.PrivateMethodStruct
	pErr := errors.New("error")
	patches := ApplyPrivateMethod(s, "lastError", func() error {
		return pErr
	})
	defer patches.Reset()
	result := f.LastError()
	So(result, ShouldEqual, pErr)
})

we got a panic: invalid memory address or nil pointer dereference