libnfnetlink: initialize attribute padding to resolve valgrind warnings

==12195== Syscall param socketcall.sendto(msg) points to uninitialised byte(s)
==12195==    at 0x51209C3: __sendto_nocancel (syscall-template.S:81)
==12195==    by 0x53E4D12: nfnl_send (libnfnetlink.c:391)
==12195==    by 0x53E6952: nfnl_query (libnfnetlink.c:1569)
==12195==    by 0x4E344AF: __build_send_cfg_msg.isra.1 (libnetfilter_log.c:143)
==12195==    by 0x4E34710: nflog_bind_group (libnetfilter_log.c:413)
==12195==    by 0x400CB1: main (nfulnl_test.c:77)
==12195==  Address 0x7fefff3e9 is on thread 1's stack

This patch sets to zero the padding that is included to align the
attribute payload.

Reported-by: Ivan Homoliak <xhomol11@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>