Place checks in callback routines because DCB will not always contain a reference to a router session, and the associated data will be invalid in this case.

server/modules/routing/readwritesplit/readwritesplit.c

@@ -71,6 +71,7 @@ extern __thread log_info_t tls_log_info;
  * 17/07/2014	Massimiliano Pinto	Server connection counter is updated in closeSession
  * 
  * 09/09/2015   Martin Brampton         Modify error handler
+ * 25/09/2015   Martin Brampton         Block callback processing when no router session in the DCB
  *
  * @endverbatim
  */
@@ -5308,6 +5309,15 @@ static int router_handle_state_switch(
         int                rc = 1;
         SERVER*            srv;
         CHK_DCB(dcb);
+    if (NULL == dcb->session->router_session)
+    {
+        /*
+         * The following processing will fail if there is no router session,
+         * because the "data" parameter will not contain meaningful data,
+         * so we have no choice but to stop here.
+         */
+        return;
+    }
         bref = (backend_ref_t *)data;
         CHK_BACKEND_REF(bref);