boardd: return early from usb functions if not connected

selfdrive/boardd/panda.cc

@@ -105,6 +105,10 @@ int Panda::usb_write(uint8_t bRequest, uint16_t wValue, uint16_t wIndex, unsigne
   int err;
   const uint8_t bmRequestType = LIBUSB_ENDPOINT_OUT | LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_RECIPIENT_DEVICE;
 
+  if (!connected){
+    return LIBUSB_ERROR_NO_DEVICE;
+  }
+
   pthread_mutex_lock(&usb_lock);
   do {
     err = libusb_control_transfer(dev_handle, bmRequestType, bRequest, wValue, wIndex, NULL, 0, timeout);
@@ -134,6 +138,10 @@ int Panda::usb_bulk_write(unsigned char endpoint, unsigned char* data, int lengt
   int err;
   int transferred = 0;
 
+  if (!connected){
+    return 0;
+  }
+
   pthread_mutex_lock(&usb_lock);
   do {
     // Try sending can messages. If the receive buffer on the panda is full it will NAK
@@ -156,6 +164,10 @@ int Panda::usb_bulk_read(unsigned char endpoint, unsigned char* data, int length
   int err;
   int transferred = 0;
 
+  if (!connected){
+    return 0;
+  }
+
   pthread_mutex_lock(&usb_lock);
 
   do {