Check if edit relates to existing endpoint

[agrrh]

Hey there!

There is note in sixnines.rb file about possibility to create new endpoint without payment:
https://github.com/yegor256/sixnines/blob/8bdbd70/sixnines.rb#L346-L353

I was able to reproduce this by using following request:

http --form POST https://www.sixnines.io/a/edit \
  'Cookie:glogin=3YBx....jYvJ' \
  old='https://example.org' \
  new='https://agrrh.com'

Result is new endpoint was created:

BTW, I was trying to remove endpoint afterwards, but it stays there w/o any error messages. Feel free to wipe my account.

[0crat]

@agrrh/z this pull request is too small, just 8 lines changed (less than 10), there will be no formal code review, see §53 and §28; in the future, try to make sure your pull requests are not too small; @null/z please review this and merge or reject

[agrrh]

@yegor256 May I ask you to take a look at this PR?

[agrrh]

@yegor256
Sorry for mentioning you directly, but I'm afraid my website is still there.