Fix memory leak when freeing the DTLS record layer

Fix memory leak when freeing the DTLS record layer

We need to check whether the sent_messages has actually buffered any
messages in it. If not we won't free the old record layer later when we
clear out the old buffered messages and a memory leak will result.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19586)