Handle nil nd_parent in spot_colon2

[jeremyevans]

This fixes an issue in Sequel's tests on ruby 3.1.0-preview1, where a
test was showing the wrong output in an exception message. The
related code rescues NameError to provide more information, then
reraises. Here's the output of Sequel's test without changes to
error_highlight:

  1) Failure:
Sequel::Model::Associations::AssociationReflection::#associated_class#test_0006_should include association inspect output if an exception would be raised [/data/code/sequel/spec/model/associ
ation_reflection_spec.rb:75]:
Expected "undefined method `last_lineno' for nil:NilClass\n\n      if nd_parent.last_lineno == @node.last_lineno\n                  ^^^^^^^^^^^^" to include # encoding: UTF-8
"#<Sequel::Model::Associations::ManyToOneAssociationReflection #<Class:0x000002594112e458>.many_to_one :c>".

This change makes the Sequel test pass. There probably is a better
fix.

I tried to reproduce this outside of Sequel's tests in order to create
a test for error_highlight, but I wasn't able to do so quickly.

[mame]

Thank you for your report. I think I got a clue. The following code will trigger an unknown bug of error_highlight (or RubyVM::AST or something).

module Foo
  Object.module_eval("::Bar", __FILE__, __LINE__)
end

The error message changes if error_highlight is enabled:

$ ruby --disable-error_highlight t.rb
t.rb:2:in `<module:Foo>': uninitialized constant Bar (NameError)
        from t.rb:2:in `module_eval'
        from t.rb:2:in `<module:Foo>'
        from t.rb:1:in `<main>'

$ ruby t.rb
t.rb:2:in `<module:Foo>': NameError
        from t.rb:2:in `module_eval'
        from t.rb:2:in `<module:Foo>'
        from t.rb:1:in `<main>'

I'll continue to investigate the issue tonight

[mame]

@jeremyevans I think the issue was fixed by the combination of d2140d7 and ruby/ruby@6a51c3e. Could you try ruby/ruby master with your code base? Let me know if it is not fixed yet.

[mame]

Ah, no, the issue is not fixed yet. Continue to investigate...

[mame]

Argh, a pilot error. I used the older ruby to check if the issue is fixed or not. I think it's fixed after all.

[jeremyevans]

I've built and installed ruby-head locally and checked and confirmed the problem is fixed. Thank you very much for fixing this!