net_sched: netem: do not call qdisc_drop() with a NULL skb

If skb_unshare() fails, we call qdisc_drop() with a NULL skb, which
is no longer supported.

Fixes: 520ac30f

 ("net_sched: drop packets after root qdisc lock is released")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>