staging: vt6655: device_rx_srv check sk_buff is NULL

There is a small chance that pRD->pRDInfo->skb could go NULL
while the interrupt is processing.

Put NULL check on loop to break out.
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6655/device_main.c

@@ -805,6 +805,10 @@ static int device_rx_srv(struct vnt_private *pDevice, unsigned int uIdx)
 	     pRD = pRD->next) {
 		if (works++ > 15)
 			break;
+
+		if (!pRD->pRDInfo->skb)
+			break;
+
 		if (vnt_receive_frame(pDevice, pRD)) {
 			if (!device_alloc_rx_buf(pDevice, pRD)) {
 				dev_err(&pDevice->pcid->dev,