Fix potential NULL pointer dereference in final_maxfragmentlen()

Fix potential NULL pointer dereference in final_maxfragmentlen()

In the final_maxfragmentlen() function, s->session is checked
for NULL after it was dereferenced earlier.
So move this NULL check to the top of the function.

CLA: trivial

Fixes: fa49560451 (Fix handling of max_fragment_length extension for PSK)
Signed-off-by: Andrey Tsygunka <aitsygunka@yandex.ru>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
(Merged from https://github.com/openssl/openssl/pull/27272)