fix: panic when no matches occur in multi-line comment

[ianlewis]

Description:

Fixes panic when no matches occur in multi-line comment.

Related Issues:

Fixes #776

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Add a note in the CHANGELOG.md if applicable.
• Sign the Google CLA.

[codecov]

Codecov Report

Merging #777 (7288ff4) into main (f512fb5) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #777   +/-   ##
=======================================
  Coverage   80.50%   80.50%           
=======================================
  Files          19       19           
  Lines        1144     1144           
  Branches       14       14           
=======================================
  Hits          921      921           
  Misses        160      160           
  Partials       63       63           

Files Changed	Coverage Δ
internal/todos/todos.go	99.16% <100.00%> (ø)