ksmbd: fix null pointer dereference error in generate_encryptionkey

In the Linux kernel, the following vulnerability has been resolved: 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.