[Backport 3.6] Fix NULL argument handling in mbedtls_xxx_free() functions

[Troy-Butler]

Description

Added conditional statements to mbedtls_xxx_free() functions to handle NULL arguments. Resolves https://github.com/Mbed-TLS/mbedtls/issues/8915 Relates to PR #8983

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [x] changelog not required
• [x] 3.6 backport of https://github.com/Mbed-TLS/mbedtls/pull/8983
• [x] 2.28 backport not required
• [x] tests not required