staging: vt6655: device_rx_srv check sk_buff is NULL

commit b5eeed8c upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/staging/vt6655/device_main.c

@@ -1402,6 +1402,10 @@ static int device_rx_srv(PSDevice pDevice, unsigned int uIdx) {
 	     pRD = pRD->next) {
 		if (works++ > 15)
 			break;
+
+		if (!pRD->pRDInfo->skb)
+			break;
+
 		if (device_receive_frame(pDevice, pRD)) {
 			if (!device_alloc_rx_buf(pDevice, pRD)) {
 				DBG_PRT(MSG_LEVEL_ERR, KERN_ERR