test for toplevel line after popping nested var

test for toplevel line after popping nested var

After reaching the end of a sequence of CNF siblings at nesting
level > 1, the parser crashes because the updated current line
is a toplevel line:

    234  FIREWALL_NETGROUP,1: "i2n"
    235     (234) FIREWALL_NETGROUP_NETWORK,0: ""
    236        (235) FIREWALL_NETGROUP_NETWORK_IP,0: "172.16.1.0"
    237        (235) FIREWALL_NETGROUP_NETWORK_NETMASK,0: "255.255.255.0"
    238  FIREWALL_NETGROUP,2: "drift-extern" # <----------------------------------- boom!
    239     (238) FIREWALL_NETGROUP_NETWORK,0: "drift"
    240        (239) FIREWALL_NETGROUP_NETWORK_IP,0: "172.16.1.85"
    241        (239) FIREWALL_NETGROUP_NETWORK_NETMASK,0: "255.255.255.255"

Fix by testing for parent lines and exiting immediately.