staging: comedi: ni_usb6501: Fix possible double-free of ->usb_rx_buf

`ni6501_alloc_usb_buffers()` is called from `ni6501_auto_attach()` to
allocate RX and TX buffers for USB transfers.  It allocates
`devpriv->usb_rx_buf` followed by `devpriv->usb_tx_buf`.  If the
allocation of `devpriv->usb_tx_buf` fails, it frees
`devpriv->usb_rx_buf`, leaving the pointer set dangling, and returns an
error.  Later, `ni6501_detach()` will be called from the core comedi
module code to clean up.  `ni6501_detach()` also frees both
`devpriv->usb_rx_buf` and `devpriv->usb_tx_buf`, but
`devpriv->usb_rx_buf` may have already beed freed, leading to a
double-free error.  Fix it bu removing the call to
`kfree(devpriv->usb_rx_buf)` from `ni6501_alloc_usb_buffers()`, relying
on `ni6501_detach()` to free the memory.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_usb6501.c

@@ -463,10 +463,8 @@ static int ni6501_alloc_usb_buffers(struct comedi_device *dev)
 
 	size = usb_endpoint_maxp(devpriv->ep_tx);
 	devpriv->usb_tx_buf = kzalloc(size, GFP_KERNEL);
-	if (!devpriv->usb_tx_buf) {
-		kfree(devpriv->usb_rx_buf);
+	if (!devpriv->usb_tx_buf)
 		return -ENOMEM;
-	}
 
 	return 0;
 }