OP_ENTER fix segmentation fault when use post arguments

### segmentation fault

OP_ENTER have possibility of segmentation fault.
This is reproduce `value_move()` use minus index or count when use post arguments.

```ruby
pr = Proc.new{|a,*b,c|
  [a,b,c]
}
pr.call() #=> segmentation fault
```

### variable assignment

And also have different spec with CRuby for variable assignment when argument is single array.

```ruby
pr = proc{|a,b,c|
  [a,b,c]
}
p pr.call([])
p pr.call([1])
```

CRuby

```ruby
[nil, nil, nil]
[1, nil, nil]
```

current mruby

```ruby
[[], nil, nil]
[1, nil, nil]
```


(problem to split was difficult to me.)