Correct return values for tls_construct_stoc_next_proto_neg

Correct return values for tls_construct_stoc_next_proto_neg

Return EXT_RETURN_NOT_SENT in the event that we don't send the extension,
rather than EXT_RETURN_SENT. This actually makes no difference at all to
the current control flow since this return value is ignored in this case
anyway. But lets make it correct anyway.

Follow on from CVE-2024-5535

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24716)

(cherry picked from commit e10a3a84bf73a3e6024c338b51f2fb4e78a3dee9)