src: obj: fix memleak in handle_free()

src: obj: fix memleak in handle_free()

Using limit object as example:

 # valgrind --leak-check=full nft list ruleset
 ==9937== Memcheck, a memory error detector
 ==9937== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
 ==9937== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
 ==9937== Command: nft list ruleset
 ==9937==
 table inet raw {
         limit lim1 {
                 rate 1/second
         }
 }
 ==9937==
 ==9937== HEAP SUMMARY:
 ==9937==     in use at exit: 5 bytes in 1 blocks
 ==9937==   total heap usage: 50 allocs, 49 frees, 212,065 bytes allocated
 ==9937==
 ==9937== 5 bytes in 1 blocks are definitely lost in loss record 1 of 1
 ==9937==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
 ==9937==    by 0x5C65AA9: strdup (strdup.c:42)
 ==9937==    by 0x4E720A3: xstrdup (utils.c:75)
 ==9937==    by 0x4E660FF: netlink_delinearize_obj (netlink.c:972)
 ==9937==    by 0x4E6641C: list_obj_cb (netlink.c:1064)
 ==9937==    by 0x50E8993: nftnl_obj_list_foreach (object.c:494)
 ==9937==    by 0x4E664EA: netlink_list_objs (netlink.c:1085)
 ==9937==    by 0x4E4FE82: cache_init_objects (rule.c:188)
 ==9937==    by 0x4E4FE82: cache_init (rule.c:221)
 ==9937==    by 0x4E4FE82: cache_update (rule.c:271)
 ==9937==    by 0x4E7716E: nft_evaluate (libnftables.c:406)
 ==9937==    by 0x4E778F7: nft_run_cmd_from_buffer (libnftables.c:447)
 ==9937==    by 0x40170F: main (main.c:326)

Fixes: 4756d92e517ae ("src: listing of stateful objects")
Signed-off-by: Eric Jallot <ejallot@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>