avoid range error in clang formatter when source spans multiple lines

Avoid this error in the clang formatter when the source range spans more than one line:

```Ruby
  1) Rubocop::Formatter::ClangStyleFormatter#report_file does not display offending source line if it is multiline
     Failure/Error: formatter.report_file('test', cop.offences)
     RangeError:
       #<Parser::Source::Range test 5...10> spans more than one line
     # ./lib/rubocop/formatter/clang_style_formatter.rb:20:in `block in report_file'
     # ./lib/rubocop/formatter/clang_style_formatter.rb:10:in `each'
     # ./lib/rubocop/formatter/clang_style_formatter.rb:10:in `report_file'
     # ./spec/rubocop/formatter/clang_style_formatter_spec.rb:61:in `block (3 levels) in <module:Formatter>'
```

Exception source: https://github.com/whitequark/parser/blob/5c3b7b35c7af06f0b76e725bf1da00ada69cec08/lib/parser/source/range.rb#L92 )