FIX: absolute_path may be nil for code added via instance_eval

This is critical for RC2 cause we can not work around it:

```
class A
  def initialize
    instance_eval 'def b; c; end', File.expand_path(__FILE__)
  end

  def c
    puts caller_locations[0].path.nil?
    # false
    puts caller_locations[0].absolute_path.nil?
    # true
  end
end

A.new.b
```