isisd: fix crash when entering "no ip[v6] router isis" twice

isisd: fix crash when entering "no ip[v6] router isis" twice

isisd CLI has some housekeeping code that removes the
"frr-isisd:isis" container from the interface configuration when
IS-IS is disabled for both IPv4 and IPv6 in the corresponding
interface.

The problem is that the code was checking the values of the
"ipv4-routing" and "ipv6-routing" leafs without checking if the
parent "frr-isisd:isis" container was present. So, entering "no
ip[v6] router isis" twice would cause isisd to crash since the
"frr-isisd:isis" container wouldn't be present the second time the
command is processed. Fix this.

isisd aborted: vtysh -c "configure terminal" -c "interface eth99" -c "no ip router isis WORD"
isisd aborted: vtysh -c "configure terminal" -c "interface eth99" -c "no ipv6 router isis"

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>