mnl: fix error rule reporting with missing table/chain and anonymous sets

mnl: fix error rule reporting with missing table/chain and anonymous sets

handle_merge() skips handle location initialization because set name != NULL.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7f64f1e in erec_print (octx=0x55555555d2c0, erec=0x55555555fcf0, debug_mask=0) at erec.c:95
95              switch (indesc->type) {
(gdb) bt
    buf=0x55555555db20 "add rule inet traffic-filter input tcp dport { 22, 80, 443 } accept") at libnftables.c:459
(gdb) p indesc
$1 = (const struct input_descriptor *) 0x0

Closes: http://bugzilla.opensuse.org/show_bug.cgi?id=1171321
Fixes: 086ec6f30c96 ("mnl: extended error support for create command")
Reported-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>