sch_sfb: Fix missing NULL check

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=44461



Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>