`Readline::HISTORY.delete_at(index)` not working for indices of negative values

[pingport80]

Summary

Readline::HISTORY.delete_at method not working for indices of negative values.

Steps to Reproduce

irb
require 'readline'
Readline::HISTORY.push('a')
=> ["a"]
Readline::HISTORY.push('b')
=> ["b"]
Readline::HISTORY.push('c')
=> ["c"]
>> Readline::HISTORY.delete_at(-1)
/var/lib/gems/2.7.0/gems/rb-readline-0.5.5/lib/readline.rb:458:in `delete_at': undefined method `+' for nil:NilClass (NoMethodError)

Additional Information

    def self.delete_at(index)
      if index < 0
        i += RbReadline.history_length

i variable is not defined here, which I think is causing the problem. I guess it was meant to be index.