netlink_delinearize: release expression before calling netlink_parse_concat_expr()

netlink_delinearize: release expression before calling netlink_parse_concat_expr()

netlink_get_register() clones the expression in the register. Release
this expression before calling netlink_parse_concat_expr() to
deconstruct the concatenation.

==15069==    at 0x4C2BBAF: malloc (vg_replace_malloc.c:299)
==15069==    by 0x4E79508: xmalloc (utils.c:36)
==15069==    by 0x4E795ED: xzalloc (utils.c:65)
==15069==    by 0x4E6029B: dtype_alloc (datatype.c:1073)
==15069==    by 0x4E6029B: concat_type_alloc (datatype.c:1127)
==15069==    by 0x4E6D3B3: netlink_delinearize_set (netlink.c:578)
==15069==    by 0x4E6D68E: list_set_cb (netlink.c:648)
==15069==    by 0x5F34023: nftnl_set_list_foreach (set.c:780)
==15069==    by 0x4E6D6F3: netlink_list_sets (netlink.c:669)
==15069==    by 0x4E5A7A3: cache_init_objects (rule.c:159)
==15069==    by 0x4E5A7A3: cache_init (rule.c:216)
==15069==    by 0x4E5A7A3: cache_update (rule.c:266)
==15069==    by 0x4E7E09E: nft_evaluate (libnftables.c:388)
==15069==    by 0x4E7E85B: nft_run_cmd_from_buffer (libnftables.c:428)

Reported-by: Václav Zindulka <vaclav.zindulka@tlapnet.cz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>