netfilter: xt_socket: check sk before checking for netns.

Only check for the network namespace if the socket is available.

Fixes: f5646501 ("netfilter: check if the socket netns is correct.")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>