Do not attempt to set unsettable types

[mkeeler]

Previously the following would fail:

package main

import (
   "github.com/hashicorp/go-msgpack/codec"
)

type WithMap struct {
   Map map[string]string
}

func main() {
   x := WithMap{Map: map[string]string{"foo": "bar"}}
   y := WithMap{Map: map[string]string{"foo": "baz"}}

   var buf []byte
   hnd := codec.MsgpackHandle{}
   enc := codec.NewEncoderBytes(&buf, &hnd)

   err := enc.Encode(x)
   if err != nil {
      panic(err)
   }

   dec := codec.NewDecoderBytes(buf, &hnd)
   err = dec.Decode(&y)
   if err != nil {
      panic(err)
   }
}

This PR fixes that.