s4:auth: check for user_info->mapped.account_name if it needs to be filled

s4:auth: check for user_info->mapped.account_name if it needs to be filled

mapped_state is a special hack for authenticate_ldap_simple_bind_send()
in order to avoid some additional work in authsam_check_password_internals().

But that code will be changed in the next commits, so we can simplify
the logic and only check for user_info->mapped.account_name being NULL.
As it's the important factor that user_info->mapped.account_name is
non-NULL down in the auth stack.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13879

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit c7b8c71b2b71bb9d95c33d403c4204376f443852)