T560: bugfix access to NoneType object

Commit f1bbc20716 ("Add jinja2 and ipaddrcheck to dependencies.")
introduced access to a NoneType object when the service was removed.

This happened after:
 $ delete service dns forwarding
 $ commit

File "/opt/vyatta/sbin/vyos-config-dns-forwarding.py", line 201, in <module>
  verify(c)
File "/opt/vyatta/sbin/vyos-config-dns-forwarding.py", line 164, in verify
  if not dns['interfaces']:
TypeError: 'NoneType' object is not subscriptable

1 files changed, 9 insertions, 1 deletions

diff --git a/src/conf-mode/vyos-config-dns-forwarding.py b/src/conf-mode/vyos-config-dns-forwarding.py
index 750a0c843..df273b321 100755
--- a/src/conf-mode/vyos-config-dns-forwarding.py
+++ b/src/conf-mode/vyos-config-dns-forwarding.py
@@ -160,6 +160,10 @@ def get_config():
     return dns
 
 def verify(dns):
+    # bail out early - looks like removal from running config
+    if dns is None:
+        return None
+
     if not dns['interfaces']:
         raise ConfigError('Error: DNS forwarding requires a configured listen interface!')
 
@@ -177,6 +181,10 @@ def verify(dns):
     return None
 
 def generate(dns):
+    # bail out early - looks like removal from running config
+    if dns is None:
+        return None
+
     tmpl = jinja2.Template(config_tmpl)
 
     config_text = tmpl.render(dns)
@@ -185,7 +193,7 @@ def generate(dns):
     return None
 
 def apply(dns):
-    if dns:
+    if dns is not None:
         os.system("systemctl restart pdns-recursor")
     else:
         # DNS forwarding is removed in the commit