ospf6d: Fix memory leak for `show ipv6 ospf6 zebra json`

ospf6d: Fix memory leak for `show ipv6 ospf6 zebra json`

$ for x in $(seq 1 10000); do vtysh -c 'show ipv6 ospf6 zebra json' >/dev/null; done

Before:
```
$ vtysh -c 'show memory ospf6d' | grep 'Total heap allocated'
  Total heap allocated:  26 MiB
```

After:
```
$ vtysh -c 'show memory ospf6d' | grep 'Total heap allocated'
  Total heap allocated:  2256 KiB
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>