mwifiex: add NULL check for PCIe Rx skb

We may get a NULL pointer here if skb allocation for Rx packet
was failed earlier.

Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwifiex/pcie.c

@@ -1211,6 +1211,12 @@ static int mwifiex_pcie_process_recv_data(struct mwifiex_adapter *adapter)
 		rd_index = card->rxbd_rdptr & reg->rx_mask;
 		skb_data = card->rx_buf_list[rd_index];
 
+		/* If skb allocation was failed earlier for Rx packet,
+		 * rx_buf_list[rd_index] would have been left with a NULL.
+		 */
+		if (!skb_data)
+			return -ENOMEM;
+
 		MWIFIEX_SKB_PACB(skb_data, &buf_pa);
 		pci_unmap_single(card->dev, buf_pa, MWIFIEX_RX_DATA_BUF_SIZE,
 				 PCI_DMA_FROMDEVICE);