fix segmentation fault for subscription data change

src/server/ua_subscription_datachange.c

@@ -61,7 +61,8 @@ MonitoredItem_delete(UA_Server *server, UA_MonitoredItem *monitoredItem) {
     }
 
     /* Remove the monitored item */
-    LIST_REMOVE(monitoredItem, listEntry);
+    if(monitoredItem->listEntry.le_prev != NULL)
+        LIST_REMOVE(monitoredItem, listEntry);
     UA_String_deleteMembers(&monitoredItem->indexRange);
     UA_ByteString_deleteMembers(&monitoredItem->lastSampledValue);
     UA_Variant_deleteMembers(&monitoredItem->lastValue);