mwl8k: fix NULL pointer dereference on receive out-of-memory

When we go into out-of-memory and fail to allocate skbuffs to
refill the receive ring with, rxq_process can end up running into
a receive ring entry that is marked as host-owned but doesn't have
an associated skbuff.  If this happens, we must break out of the
rx processing loop instead of trying to process the descriptor.
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwl8k.c

@@ -1012,6 +1012,8 @@ static int rxq_process(struct ieee80211_hw *hw, int index, int limit)
 		rmb();
 
 		skb = rxq->rx_skb[rxq->rx_head];
+		if (skb == NULL)
+			break;
 		rxq->rx_skb[rxq->rx_head] = NULL;
 
 		rxq->rx_head = (rxq->rx_head + 1) % MWL8K_RX_DESCS;