docs: document that *_free(NULL) does nothing

docs: document that *_free(NULL) does nothing

Explicitly documents that *_free(NULL) does nothing.
Fixes two cases where that wasn't true.
Fixes #24675.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Sasa Nedvedicky <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24735)