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>