Support unmarshal without struct

[at-wat]

return map[string]interface{} like encoding/json

[nerg4l]

Hello,

When unmarshalling a nil map causes panic assignment to entry in nil map. This means I have to initialise it before hand.

OK:

	ret := map[string]interface{}{}
	if err := ebml.Unmarshal(r, &ret); err != nil {
		fmt.Printf("error: %v\n", err)
		return
	}

Not OK:

	var ret map[string]interface{}
	if err := ebml.Unmarshal(r, &ret); err != nil {
		fmt.Printf("error: %v\n", err)
		return
	}

Lastly, it might only relates to matroska but test5.mkv from https://github.com/Matroska-Org/matroska-test-files#test-files panics reflect.Set: value of type []uint8 is not assignable to type uint8.

[at-wat]

	var ret map[string]interface{}
	if err := ebml.Unmarshal(r, &ret); err != nil {

is equivalent of

	if err := ebml.Unmarshal(r, nil); err != nil {

so I think it's invalid anyway.

encoding/json.Unmarshal in Go stdlib also don't support unmarshaling to nil map.
https://play.golang.org/p/zoXxUqVOAnT

But it's better to return error instead of causing panic.

---

encoding/json.Unmarshal seems automatically allocate map if given map is nil.
https://play.golang.org/p/H_84iLCn10r

[nerg4l]

Note: The two example is not equivalent. Both variable has nil value but their type is different.

	var ret map[string]interface{}
	fmt.Println(reflect.TypeOf(ret)) // output: map[string]interface {}
	fmt.Println(reflect.TypeOf(nil)) // output: <nil>

https://play.golang.org/p/HNYuND4iEdQ