[sinatra] sinatra does not validate expanded path matches

Sinatra before 2.2.0 does not validate that the expanded path matches public_dir when serving static files.

References

• https://nvd.nist.gov/vuln/detail/CVE-2022-29970
• https://github.com/sinatra/sinatra/pull/1683/commits/462c3ca1db53ed3cfc394cf5948e9c948ad1c10e
• https://github.com/skylightio/skylight-ruby/pull/294
• https://github.com/advisories/GHSA-qp49-3pvw-x4m5