Don't apply max_frag_len checking if no Max Fragment Length extension

Don't apply max_frag_len checking if no Max Fragment Length extension

Don't check the Max Fragment Length if the it hasn't been negotiated. We
were checking it anyway, and using the default value
(SSL3_RT_MAX_PLAIN_LENGTH). This works in most cases but KTLS can cause the
record length to actually exceed this in some cases.

Fixes #23169

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23182)