UTIL: Suppress Coverity warning
    
    We recently added this code:
             if (domain_name != NULL
                    &&  is_files_provider(find_domain_by_name(dom,
                                                              domain_name,
                                                              false)))
    
    find_domain_by_name returns NULL if the domain_name can't be found. This
    of course makes mostly sense for trusted domains that can appear and
    disappear. And is_files_provider() didn't handle the situation where the
    domain pointer was NULL and would directly dereference it.
    
    This commit just adds a NULL check for the domain pointer so that
    is_files_provider() returns 'false' if the domain pointer was NULL.
    
    Another alternative might be to check the return value of
    find_domain_by_name(), but I don't think it's worth the trouble.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>