usb: dwc3: gadget: Fix null pointer exception

usb: dwc3: gadget: Fix null pointer exception

In the endpoint interrupt functions
dwc3_gadget_endpoint_transfer_in_progress() and
dwc3_gadget_endpoint_trbs_complete() will dereference the endpoint
descriptor. But it could be cleared in __dwc3_gadget_ep_disable()
when accessory disconnected. So we need to check whether it is null
or not before dereferencing it.

Fixes: f09ddcfcb8c5 ("usb: dwc3: gadget: Prevent EP queuing while stopping transfers")
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Jack Pham <quic_jackp@quicinc.com>
Signed-off-by: Albert Wang <albertccwang@google.com>
Link: https://lore.kernel.org/r/20211109092642.3507692-1-albertccwang@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>