fix for service monitor sync issues with snat

when the snat DB is corrupted, service monitor throws
an exception and fails to sync with other snat services.

Added validity code to check the object before it is used

Change-Id: Ia512768f9d5749428256e56b5f472a4bf7cd260c
Closes-bug: 1617043

src/config/svc-monitor/svc_monitor/snat_agent.py

@@ -22,7 +22,7 @@ def audit_snat_instances(self):
         for si in ServiceInstanceSM.values():
             si_name = si.fq_name[-1]
             st_obj = ServiceTemplateSM.get(si.service_template)
-            if st_obj.params['service_type'] != "source-nat":
+            if st_obj == None or st_obj.params['service_type'] != "source-nat":
                 continue
             lr_uuid = si.logical_router
             lr = LogicalRouterSM.get(lr_uuid)