netdev-dpdk: Fix crash in QoS.

netdev-dpdk: Fix crash in QoS.

qos_conf can be NULL.  This can be easily reproduced by setting egress
QoS on a port:

```
ovs-vsctl set port dpdk2 qos=@newqos -- --id=@newqos create qos
type=egress-policer other-config:cir=46000000 other-config:cbs=2048
```

Reported-by: Ian Stokes <ian.stokes@intel.com>
Fixes: 78bd47cf44a5 ("netdev-dpdk: Use RCU for egress QoS.")
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Tested-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Ian Stokes <ian.stokes@intel.com>