Add a check for none before accessing variable

The root cause of the bug was fixed with this review:
https://review.opencontrail.org/#/c/17915/
However, adding a defensive check also in case we still hit the issue.

Change-Id: Ib17229d652c588e3698a5df6c399f8a5264b90f8
Closes-Bug: 1550156
(cherry picked from commit f279c16)

src/config/schema-transformer/config_db.py

@@ -1937,6 +1937,8 @@ def update_routing_policy_and_aggregates(self):
         if not self.service_chain:
             return
         sc = ServiceChain.get(self.service_chain)
+        if sc is None:
+            return
         for si_name in sc.service_list:
             if not self.name.endswith(si_name.replace(':', '_')):
                 continue