ksmbd: validate zero num_subauth before sub_auth is accessed

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

ksmbd: validate zero num_subauth before sub_auth is accessed

Access psid->sub_auth[psid->num_subauth - 1] without checking

if num_subauth is non-zero leads to an out-of-bounds read.

This patch adds a validation step to ensure num_subauth != 0

before sub_auth is accessed.