dhclient: check for out variable being None

Fixes: 765075ba392 ("dhclient: wait to start dhcp if carrier is down")
Signed-off-by: Andy Roulin <aroulin@nvidia.com>

ifupdown2/ifupdownaddons/dhclient.py

@@ -73,6 +73,8 @@ class dhclient(utilsBase):
         while '1' not in out and retries < 5:
             path = 'sys/class/net/%s/carrier' %ifacename
             out = self.read_file_oneline(path)
+            if out is None:
+                break # No sysfs file found for this iface
             retries += 1
             time.sleep(1)