ldap_child: initialized ccname_file_dummy
ccname_file_dummy is used in the done-block which is called before
ccname_file_dummy is set to a value. This patch initializes
ccname_file_dummy to NULL.
Related to https://fedorahosted.org/sssd/ticket/2592
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>