Panda: add check for connected in usb_read (#19957)

selfdrive/boardd/panda.cc

@@ -118,6 +118,10 @@ int Panda::usb_read(uint8_t bRequest, uint16_t wValue, uint16_t wIndex, unsigned
   int err;
   const uint8_t bmRequestType = LIBUSB_ENDPOINT_IN | LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_RECIPIENT_DEVICE;
 
+  if (!connected){
+    return LIBUSB_ERROR_NO_DEVICE;
+  }
+
   std::lock_guard lk(usb_lock);
   do {
     err = libusb_control_transfer(dev_handle, bmRequestType, bRequest, wValue, wIndex, data, wLength, timeout);