tipc: remove an unnecessary NULL check

This is never called with a NULL "buf" and anyway, we dereference 's' on
the lines before so it would Oops before we reach the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>