econet: disallow NULL remote addr for sendmsg(), fixes CVE-2010-3849

Later parts of econet_sendmsg() rely on saddr != NULL, so return early
with EINVAL if NULL was passed otherwise an oops may occur.

Signed-off-by: Phil Blundell <philb@gnu.org>
Signed-off-by: David S. Miller <davem@davemloft.net>