lib: Fix assert in node_parent()

lib: Fix assert in node_parent()

With this sequence of commands in a conf file:
router bgp X
address-family ipv4
exit
address-family ipv6
exit

When the bgp AS # is currently configured as !X and the
'vtysh -b -f <conf file above>' command run, vtysh will
crash with an assert.

This commit fixes that issue.

Ticket: CM-9852
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>