net: sched: atm: dont intepret cls results when asked to drop

If asked to drop a packet via TC_ACT_SHOT it is unsafe to assume
res.class contains a valid pointer
Fixes: b0188d4d ("[NET_SCHED]: sch_atm: Lindent")

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>