usb: handle dev == NULL in usb_handle_packet()

Allow passing in a NULL pointer, return USB_RET_NODEV in that case.
Removes the burden to to a NULL pointer check from the callers.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb.c

@@ -315,6 +315,10 @@ int usb_handle_packet(USBDevice *dev, USBPacket *p)
 {
     int ret;
 
+    if (dev == NULL) {
+        return USB_RET_NODEV;
+    }
+
     assert(p->owner == NULL);
     ret = usb_device_handle_packet(dev, p);
     if (ret == USB_RET_ASYNC) {