uhci: Don't crash on device disconnect

My recent uhci cleanup series has introduced a regression, where
qemu sometimes crashes on a device disconnect. The problem is that
the uhci code never checked for a device not / no longer existing, instead
it was relying on usb_handle_packet accepting a NULL device.

But since we now pass usb_handle_packet q->ep->dev, rather then just
a local dev variable, we crash as q->ep == NULL due to the device no longer
existing.

This patch fixes this. Note that this patch also improves over
the old behavior were we would:
1) create a queue for the device
2) create an async for the packet
3) have usb_handle_packet fail
4) destroy the async
5) wait for the queue to be idle for 32 frames
6) destroy the queue

Which was rather sub-optimal.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-uhci.c

@@ -896,6 +896,11 @@ static int uhci_handle_td(UHCIState *s, UHCIQueue *q, uint32_t qh_addr,
     if (q == NULL) {
         USBDevice *dev = uhci_find_device(s, (td->token >> 8) & 0x7f);
         USBEndpoint *ep = usb_ep_get(dev, pid, (td->token >> 15) & 0xf);
+
+        if (ep == NULL) {
+            return uhci_handle_td_error(s, td, td_addr, USB_RET_NODEV,
+                                        int_mask);
+        }
         q = uhci_queue_new(s, qh_addr, td, ep);
     }
     async = uhci_async_alloc(q, td_addr);