ksmbd: fix null pointer dereference error in generate_encryptionkey

If client send two session setups with krb5 authenticate to ksmbd,
null pointer dereference error in generate_encryptionkey could happen.
sess->Preauth_HashValue is set to NULL if session is valid.
So this patch skip generate encryption key if session is valid.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-27654
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>