driver core: auxiliary bus: Fix auxiliary bus shutdown null auxdrv ptr

If the probe of the auxdrv failed, the device->driver is set to NULL.
During kernel shutdown, the bus shutdown will call auxdrv->shutdown and
cause an invalid ptr dereference. Add check to make sure device->driver is
not NULL before we proceed.

Fixes: 7de3697e ("Add auxiliary bus support")
Cc: Dave Ertman <david.m.ertman@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/160710040926.1889434.8840329810698403478.stgit@djiang5-desk3.ch.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/auxiliary.c

@@ -92,10 +92,15 @@ static int auxiliary_bus_remove(struct device *dev)
 
 static void auxiliary_bus_shutdown(struct device *dev)
 {
-	struct auxiliary_driver *auxdrv = to_auxiliary_drv(dev->driver);
-	struct auxiliary_device *auxdev = to_auxiliary_dev(dev);
+	struct auxiliary_driver *auxdrv = NULL;
+	struct auxiliary_device *auxdev;
+
+	if (dev->driver) {
+		auxdrv = to_auxiliary_drv(dev->driver);
+		auxdev = to_auxiliary_dev(dev);
+	}
 
-	if (auxdrv->shutdown)
+	if (auxdrv && auxdrv->shutdown)
 		auxdrv->shutdown(auxdev);
 }