[Bug] reflect: slice index out of range at Copy of a slice of pointers to struct

## Reproducible Example

https://play.golang.org/p/XPEKi-yqqFA

## Description

Hi everyone,

I'm facing an error using the Copy() method when I copy an interface{} (real type : slice of pointers to struct) to an other interface{}.

See above the example.

This test case was correct in version v0.1.0 and is wfrom from version v0.2.0 to v0.2.5

@jinzhu
