ixgbe: Fix possible null-dereference in error path

commit 508a8c9e upstream.

In ixgbe_probe, the code at label err_dma can dereference adapter
when it has a NULL value. The check is there to avoid disabling a
disabled device. When adapter is NULL, treat it as if the device
is enabled, because it is enabled in that case.
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -8477,7 +8477,7 @@ static int ixgbe_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 				     pci_select_bars(pdev, IORESOURCE_MEM));
 err_pci_reg:
 err_dma:
-	if (!test_and_set_bit(__IXGBE_DISABLED, &adapter->state))
+	if (!adapter || !test_and_set_bit(__IXGBE_DISABLED, &adapter->state))
 		pci_disable_device(pdev);
 	return err;
 }