Handle invalid characters in Style/PercentLiteralDelimiters

[eugeneius]

Previously this would crash with:

ArgumentError: invalid byte sequence in UTF-8
lib/rubocop/cop/style/percent_literal_delimiters.rb:105:in `match?'

I discovered this problem by running RuboCop on the real-world-rails repo, which includes the discourse repo:

https://github.com/discourse/discourse/blob/544a9865c6e5890f37e762292cbe5558db843dcf/spec/helpers/application_helper_spec.rb#L263

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Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

Thanks!