netxen: check for root bus in netxen_mask_aer_correctable

Add a check if pdev->bus->self == NULL (root bus). When attaching
a netxen NIC to a VM it can be on the root bus and the guest would
crash in netxen_mask_aer_correctable() because of a NULL pointer
dereference if CONFIG_PCIEAER is present.
Signed-off-by: NNikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/qlogic/netxen/netxen_nic_main.c

@@ -1378,6 +1378,10 @@ static void netxen_mask_aer_correctable(struct netxen_adapter *adapter)
 	struct pci_dev *root = pdev->bus->self;
 	u32 aer_pos;
 
+	/* root bus? */
+	if (!root)
+		return;
+
 	if (adapter->ahw.board_type != NETXEN_BRDTYPE_P3_4_GB_MM &&
 		adapter->ahw.board_type != NETXEN_BRDTYPE_P3_10G_TP)
 		return;