In the past code, the current `callinfo (ci)` was modified, thus it was
possible to pop `ci` beyond the `cibase`, that could cause out of memory
bound access for the code like the following:

```ruby
def m2
  lambda {
    Proc.new {
      return :return # return from the method
    }
  }.call.call
  :never_reached
end

p m2
```