netfilter: nft_socket: Break evaluation if no socket found

Actual implementation stores 0 in the destination register if no socket
is found by the lookup, but that is not intentional as it is not really
a value of any socket metadata.

This patch fixes this and breaks rule evaluation in this case.

Fixes: 554ced0a

 ("netfilter: nf_tables: add support for native socket matching")
Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>