[Fix #8590] Fix an error when auto-correcting encoding mismatch file

[koic]

Fixes #8590.

This PR fixes the following error when auto-correcting encoding mismatch file.

% cat example.rb
# encoding: Shift_JIS

puts 'Ｔｈｉｓ ｆｉｌｅ ｅｎｃｏｄｉｎｇ ｉｓ ＵＴＦ－８．'

Before

% bundle exec rubocop --cache false -a
(snip)

Cannot extract source from uninitialized Source::Buffer
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/parser-2.7.1.4/lib/
parser/source/buffer.rb:148:in `source'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/parser-2.7.1.4/lib/
parser/source/buffer.rb:297:in `source_range'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/parser-2.7.1.4/lib/
parser/source/tree_rewriter.rb:116:in `initialize'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/corrector.rb:18:in
`initialize'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/team.rb:182:in
`new'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/team.rb:182:in
`collate_corrections'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/team.rb:176:in
`autocorrect_report'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/team.rb:121:in
`autocorrect'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/team.rb:79:in
`investigate'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/runner.rb:295:in `inspect_file'

After

% bundle exec rubocop --cache false -a
(snip)

Inspecting 1 file
F

Offenses:

example.rb:1:1: F: Lint/Syntax: "\xef\xbc" from shift_jis to utf-8.

1 file inspected, 1 offense detected

---

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.

[marcandre]

Great, thanks!