Fixes the following code scanning alerts.

• https://github.com/google/certificate-transparency-go/security/code-scanning/14
• https://github.com/google/certificate-transparency-go/security/code-scanning/16
• https://github.com/google/certificate-transparency-go/security/code-scanning/17
• https://github.com/google/certificate-transparency-go/security/code-scanning/18
• https://github.com/google/certificate-transparency-go/security/code-scanning/19
• https://github.com/google/certificate-transparency-go/security/code-scanning/20
• https://github.com/google/certificate-transparency-go/security/code-scanning/21
• https://github.com/google/certificate-transparency-go/security/code-scanning/28

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly.