Invalid @bind_resolver after bind_prefix_key with a 2nd arg

rbindkeys should not raise in the belowing code:

``` ruby
br = BindResolver.new @default_bind_resolver, false
current_bind_resolver = @bind_resolver
bind_prefix_key [KEY_LEFTCTRL, KEY_Q], br do
  bind_key [KEY_LEFTCTRL,KEY_D], KEY_END
end
if @bind_resolver != current_bind_resolver
  raise "unexpected state"
end
```
