Fix panic threat when using type-cast for customClaims in validator

[osamingo]

Description

The threat of panicking should be curbed by receiving a second return value when executing the type-cast.

References

• CodeReviewComments: https://github.com/golang/go/wiki/CodeReviewComments#dont-panic
• Effective Go: https://go.dev/doc/effective_go#interface_conversions

Testing

• [x] This change adds test coverage for new/changed/fixed functionality

Checklist

• [x] I have read and agreed to the terms within the Auth0 Code of Conduct.
• [x] I have read the Auth0 General Contribution Guidelines.
• [x] I have reviewed my own code beforehand.
• [x] I have added documentation for new/changed functionality in this PR.
• [x] All active GitHub checks for tests, formatting, and security are passing.
• [ ] The correct base branch is being used, if not master.

[osamingo]

I would appreciate it if you would approve running GitHub Actions for this PR when you have time.

[osamingo]

@Widcket ping

[osamingo]

@sergiught I fixed it, PTAL.

[codecov-commenter]

Codecov Report

Base: 89.05% // Head: 89.09% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (2bc0d47) compared to base (e9dbfc6). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #165 +/- ## ========================================== + Coverage 89.05% 89.09% +0.03% ========================================== Files 7 7 Lines 274 275 +1 ========================================== + Hits 244 245 +1 Misses 23 23 Partials 7 7 ``` | [Impacted Files](https://codecov.io/gh/auth0/go-jwt-middleware/pull/165?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [validator/validator.go](https://codecov.io/gh/auth0/go-jwt-middleware/pull/165/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#diff-dmFsaWRhdG9yL3ZhbGlkYXRvci5nbw==) | `88.75% <100.00%> (+0.14%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0)

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[sergiught]

Hey @osamingo 👋🏻 thanks a lot for the changes and apologies it took so while to get back to you! Would you mind adding a test case for this as well in the validatior_test.go file? 🙏🏻

[sergiught]

Hey @osamingo 👋🏻 I had some time and went ahead and added the test cases missing. Thanks for the contribution again!