Gracefully handle fetch network fail in qos extend port

When the qos plugin is handling a port resource request through it's
port resource request extension, sometimes the network a port is
attached to is looked up and returns None. It may happen like that
if network will be deleted in concurrent API request.

Change-Id: Ide4acdf4c373713968f9d43274fb0c7550283c11
Closes-Bug: #1810504

neutron/services/qos/qos_plugin.py

@@ -99,7 +99,7 @@ class QoSPlugin(qos.QoSPluginBase):
         if not qos_policy:
             net = network_object.Network.get_object(
                 context.get_admin_context(), id=port_res['network_id'])
-            if net.qos_policy_id:
+            if net and net.qos_policy_id:
                 qos_policy = policy_object.QosPolicy.get_network_policy(
                     context.get_admin_context(), net.id)