[SMF] Allow Gy CCA event to contain NULL gtp_xact

This happens for instance when the session is terminated due to a
rejection coming from the OCS, hence no originating GTP xact producing
the tear down.

src/smf/gsm-sm.c

@@ -1486,7 +1486,6 @@ void smf_gsm_state_wait_epc_auth_release(ogs_fsm_t *s, smf_event_t *e)
         case OGS_DIAM_GY_CMD_CODE_CREDIT_CONTROL:
             switch(gy_message->cc_request_type) {
             case OGS_DIAM_GY_CC_REQUEST_TYPE_TERMINATION_REQUEST:
-                ogs_assert(e->gtp_xact);
                 diam_err = smf_gy_handle_cca_termination_request(sess,
                                 gy_message, e->gtp_xact);
                 sess->sm_data.gy_ccr_term_in_flight = false;