Fix calling exit if irb_context is nil

[k0kubun]

ruby/ruby master is randomly experiencing this error:

  D:/a/ruby/ruby/src/lib/irb/extend-command.rb:25:in `irb_exit': private method `exit' called for nil (NoMethodError)
  
        irb_context.exit(ret)
                   ^^^^^
  	from D:/a/ruby/ruby/src/tool/test/runner.rb:23:in `<top (required)>'
  	from ../src/test/runner.rb:16:in `require_relative'
  	from ../src/test/runner.rb:16:in `<main>'

I'm not sure how to deterministically reproduce this issue (I suspect there's a loading order that could mess this up), but since it's failing on a nil receiver, we could address this by checking if the receiver is nil.

[st0012]

Thanks. I will take a look