Fix reading of nested cnfvars from string

Fix reading of nested cnfvars from string

The function parse_cnf_children previously disregarded the children
received from a recursive parse_cnf_children call if the state was None
after the recursive function call. Hence, the leftover children would
be omitted instead of attached to the corresponding parent cnfvar. This
lead to the following configuration omitting the _IP and _NETMASK
entries:

1 FIREWALL_NETGROUP,99: "QA host IP"
2   (1) FIREWALL_NETGROUP_NETWORK,0: ""
3     (2) FIREWALL_NETGROUP_NETWORK_IP,0: "192.168.1.254"
4     (2) FIREWALL_NETGROUP_NETWORK_NETMASK,0: "255.255.255.255"