thunderbolt: Fix possible NULL pointer dereference in tb_acpi_add_link()

When we walk up the device hierarchy in tb_acpi_add_link() make sure we
break the loop if the device has no parent. Otherwise we may crash the
kernel by dereferencing a NULL pointer.

Fixes: b2be2b05 ("thunderbolt: Create device links from ACPI description")
Cc: stable@vger.kernel.org
Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/acpi.c

@@ -56,7 +56,7 @@ static acpi_status tb_acpi_add_link(acpi_handle handle, u32 level, void *data,
 	 * managed with the xHCI and the SuperSpeed hub so we create the
 	 * link from xHCI instead.
 	 */
-	while (!dev_is_pci(dev))
+	while (dev && !dev_is_pci(dev))
 		dev = dev->parent;
 
 	if (!dev)