ospf6d: Prevent crash of show ipv6 ospf data adv-router 0.0.0.0 linkstate-id 0.0.0.0

ospf6d: Prevent crash of show ipv6 ospf data adv-router 0.0.0.0 linkstate-id 0.0.0.0

With this sequence of events:
eva# conf
eva(config)# router ospf6
eva(config-ospf6)# end
eva# show ipv6 ospf data adv-router 0.0.0.0 linkstate-id 0.0.0.0
OSPF6: Received signal 11 at 1630442431 (si_addr 0x0, PC 0x559dcfa3a656); aborting...
OSPF6: zlog_signal+0x18c                  7fd2cc8229f7     7fff606775d0 /lib/libfrr.so.0 (mapped at 0x7fd2cc770000)
OSPF6: core_handler+0xe3                  7fd2cc8616ad     7fff606776f0 /lib/libfrr.so.0 (mapped at 0x7fd2cc770000)
OSPF6: funlockfile+0x50                   7fd2cc74f140     7fff60677840 /lib/x86_64-linux-gnu/libpthread.so.0 (mapped at 0x7fd2cc73b000)
OSPF6:     ---- signal ----
OSPF6: ospf6_lsdb_type_show_wrapper+0x5d     559dcfa3a656     7fff60677dd0 /usr/lib/frr/ospf6d (mapped at 0x559dcf9a5000)
OSPF6: show_ipv6_ospf6_database_adv_router_linkstate_id+0x1f9     559dcfa3c24a     7fff60677e50 /usr/lib/frr/ospf6d (mapped at 0x559dcf9a5000)

OSPF6 crashes.  Fix.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>