staging: vt6655: device_rx_srv check sk_buff is NULL

commit b5eeed8cb6097c8ea660b6598d36fdbb94065a22 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>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/staging/vt6655/device_main.c
Simple merge