In queue.c from FreeRTOS 8.2.2 the xQueueGenericCreate function ends (see line 383 on) with:
configASSERT( xReturn ); return xReturn;
so it’s impossible for the function to return NULL without triggering the assertion. However the documentation (http://www.freertos.org/a00116.html) states that xQueueCreate can return 0 to indicate failure.
If assertions were turned off this function would operate as documented, so it seems like this assert should not be present.