ipapwd_pre_mod: NULL ptr deref
    
    In ipapwd_pre_mod, check userpw for NULL before dereferencing its first
    element.
    
    See: https://pagure.io/freeipa/issue/7738
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>