dyndns: Add checks for NULL
    
    Fixes:
    https://fedorahosted.org/sssd/ticket/3076
    
    We segfaulted in this area once. This patch
    makes the code more defensive and adds
    some DEBUG messages.
    
    Normally the structures are filled in online
    and/or resolve callbacks.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>