smb: Rework anonymous handling to avoid EINVAL
After the recent samba change, the "Invalid Argument" error can be still
returned when anonymous login is requested even after the commit 747c7f6e.
This is because smbc_setOptionNoAutoAnonymousLogin
is called after returning
from the auth_callback
function (i.e. there is one redundant iteration).
Let's rework the handling a bit and call that immediately, which bypasses
the issue.
Fixes: #619 (closed)