fib: Fix crash on cover update to non activated adj source

if the adj source is not active then there is no existing cover
during a cover update

Type: fix
Ticket: VPP-1803
Signed-off-by: Neale Ranns <nranns@cisco.com>
Change-Id: Ie912f1c99345de4fb012bdcd97b443098d4a7351
diff --git a/test/vpp_ip_route.py b/test/vpp_ip_route.py
index 031412c..3ac0e84 100644
--- a/test/vpp_ip_route.py
+++ b/test/vpp_ip_route.py
@@ -246,6 +246,7 @@
             sw_if_index=self.intf.sw_if_index, prefix=self.prefix,
             is_add=1)
         self._test.registry.register(self, self._test.logger)
+        return self
 
     def remove_vpp_config(self):
         self._test.vapi.sw_interface_add_del_address(