netlabel: Fix NULL pointer exception caused by CALIPSO on IPv4 sockets

In the Linux kernel, the following vulnerability has been resolved:

netlabel: Fix NULL pointer exception caused by CALIPSO on IPv4 sockets

When calling netlbl_conn_setattr(), addr->sa_family is used

to determine the function behavior. If sk is an IPv4 socket,

but the connect function is called with an IPv6 address,

the function calipso_sock_setattr() is triggered.

Inside this function, the following code is executed:

sk_fullsock(__sk) ? inet_sk(__sk)->pinet6 : NULL;

Since sk is an IPv4 socket, pinet6 is NULL, leading to a

null pointer dereference.

This patch fixes the issue by checking if inet6_sk(sk)

returns a NULL pointer before accessing pinet6.