NSX|V3: ensure that variables are assigned

If router_update fails with a ManagerError then the exception
handling may use an unassigned variable.

Change-Id: I46f44dea7e08c5c5108480ddd0ec449f006aca7e

vmware_nsx/plugins/nsx_v3/plugin.py

@@ -2314,6 +2314,8 @@ class NsxV3Plugin(agentschedulers_db.AZDhcpAgentSchedulerDbMixin,
         gw_info = self._extract_external_gw(context, router, is_extract=False)
         router_data = router['router']
         nsx_router_id = None
+        routes_added = []
+        routes_removed = []
         try:
             if 'routes' in router_data:
                 new_routes = router_data['routes']