tls13/finished: addressed memory leak in receiving finished packet

This addresses a memory leak when receiving a finished packet under TLS1.3.

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7518

Checklist

• Code modified for feature
• Test suite updated with negative tests

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code