nimble/ll: Fix assert on aux unref during scan

This fixes assert in ble_ll_scan_aux_data_unref() during scan under
heavy load. This happens when we scanned an aux with auxptr, but we
failed to schedule scan for pointed aux.

What happens in above scenario is that scan error is already set due
to failed scheduling, but we still send complete received aux. That
is ok, but we do not check for error that is already set and do not
mark data as truncated which results in an assert: if any data were
sent to host, it's expected that we'll have either truncated or
completed flag set on last unref.

The fix is to set truncated flag properly on last chunk of data from
mentioned aux.

nimble/controller/src/ble_ll_scan.c

@@ -2687,10 +2687,16 @@ ble_ll_hci_send_ext_adv_report(uint8_t ptype, uint8_t *adva, uint8_t adva_type,
 
         /*
          * We need another event if either there are still some data left to
-         * send in current PDU or scan is not completed. The only exception is
-         * when this is a scannable event which is not a scan response.
+         * send in current PDU or scan is not completed. There are two exceptions
+         * though:
+         * - we sent all data from this PDU and there is scan error set already;
+         *   it may be set before entering current function due to failed aux
+         *   scan scheduling
+         * - this is a scannable event which is not a scan response
          */
-        need_event = ((offset < datalen) || (aux_data && !(aux_data->flags_ll & BLE_LL_AUX_FLAG_SCAN_COMPLETE))) && !is_scannable_aux;
+        need_event = ((offset < datalen) || (aux_data && !(aux_data->flags_ll & BLE_LL_AUX_FLAG_SCAN_COMPLETE))) &&
+                     !((offset == datalen) && (aux_data->flags_ll & BLE_LL_AUX_FLAG_SCAN_ERROR)) &&
+                     !is_scannable_aux;
 
         if (need_event) {
             /*