fixes #39 - panic for corrupt private keys in tls_public_key

[harryrose]

Provides a fix + tests for a panic that occurs when the private key pem is corrupt

[apparentlymart]

Thanks @harryrose!

I have removed the CHANGELOG part of your changeset because we generally do that only after merging so that the CHANGELOG doesn't become conflicted immediately after each merge, but I'm otherwise going to merge this as-is once the tests finish running.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.