(h ||= {})[k]

Fixes for:

```ruby
 > Parser::CurrentRuby.parse('(h ||= {})[k]')
=> (send
  (begin
    (or-asgn
      (lvasgn :h)
      (hash))) :[]
  (send nil :k))

 > ast = Parser::CurrentRuby.parse("(h ||= {})[k] = v")
=> (send
  (begin
    (or-asgn
      (lvasgn :h)
      (hash))) :[]=
  (send nil :k)
  (send nil :v))
```