VOL-1019 check port exists before updating stats

Change-Id: I416d3b13999c8edc51cc63b2c298d3fa7ce70603
diff --git a/voltha/adapters/openolt/openolt_device.py b/voltha/adapters/openolt/openolt_device.py
index 97062a3..ac740eb 100644
--- a/voltha/adapters/openolt/openolt_device.py
+++ b/voltha/adapters/openolt/openolt_device.py
@@ -690,6 +690,10 @@
         port = self.adapter_agent.get_port(self.device_id,
             port_no=port_stats.intf_id)
 
+        if port is None:
+            self.log.warn('port associated with this stats does not exist')
+            return 
+
         port.rx_packets = port_stats.rx_packets
         port.rx_bytes = port_stats.rx_bytes
         port.rx_errors = port_stats.rx_error_packets