Fix an error for Layout/LineLength

[koic]

This PR fixes the following infinite loop error for Layout/LineLength when over limit at right hand side of multiple assignment.

% cat .rubocop.yml
Layout/LineLength:
  Max: 40

% cat example.rb
a = fooooooooooooooooooooooooooooooooooooo, b

% bundle exec rubocop --only Layout/LineLength -a
(snip)

0 files inspected, 200 offenses detected, 200 offenses corrected
Infinite loop detected in /tmp/example.rb and caused by Layout/LineLength
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/runner.rb:290:in
`block in iterate_until_no_changes'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/runner.rb:286:in
`loop'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/runner.rb:286:in
`iterate_until_no_changes'

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!