tipc: correctly handle releasing a not fully initialized sock

Commit f2f9800d "tipc: make tipc node table aware of net
namespace" has added a dereference of sock->sk before making sure it's
not NULL, which makes releasing a tipc socket NULL pointer dereference
for sockets that are not fully initialized.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>