netfilter: nfnetlink_queue: Initialize ctx to avoid memory allocation error

It is possible that ctx in nfqnl_build_packet_message() could be used
before it is properly initialize, which is only initialized
by nfqnl_get_sk_secctx().

This patch corrects this problem by initializing the lsmctx to a safe
value when it is declared.

This is similar to the commit 35fcac7a
("audit: Initialize lsmctx to avoid memory allocation error").

Fixes: 2d470c77 ("lsm: replace context+len with lsm_context")
Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>