Move freeing of an old enc_write_ctx/write_hash to dtls1_clear_sent_buffer

Move freeing of an old enc_write_ctx/write_hash to dtls1_clear_sent_buffer

When we are clearing the sent messages queue we should ensure we free any
old enc_write_ctx/write_hash that are no longer in use. Previously this
logic was in dtls1_hm_fragment_free() - but this can end up freeing the
current enc_write_ctx/write_hash under certain error conditions.

Fixes #22664

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2261)