Since endpoint 0 is used for control requests, it doesn't have a class driver attached to it. As such, the corresponding `_usbd_dev.ep2drv` entry points to driver `0xFF`, which is invalid and this makes the `TU_ASSERT(drvid < USBD_CLASS_DRIVER_COUNT);` line fail, and eventually causes an endpoint stall. So as-is the stack cannot respond to any endpoint requests on endpoint 0.
However, standard requests on endpoint 0 do not need a class driver to produce a valid response. This commit changes the order of execution so that the assert is only checked if the endpoint is not 0.
Jeremy Herbert