Fix handling of max_fragment_length extension for PSK

Fix handling of max_fragment_length extension for PSK

A psk session was assumed to be a resumption which failed a check
when parsing the max_fragment_length extension hello from the client.

Relevant code from PR#18130 which was a suggested fix to the issue
was cherry-picked.

Fixes #18121

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24513)

(cherry picked from commit fa495604516a610d988f02298c8d97a6ac4777bb)