usb: host: max3421-hcd: Fix error pointer dereference in probe cleanup

The kthread_run() function returns error pointers so the
max3421_hcd->spi_thread pointer can be either error pointers or NULL.
Check for both before dereferencing it.

Fixes: 05dfa5c9bc37 ("usb: host: max3421-hcd: fix "spi_rd8" uses dynamic stack allocation warning")
Signed-off-by: Dan Carpenter <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/host/max3421-hcd.c

···
 	if (hcd) {
 		kfree(max3421_hcd->tx);
 		kfree(max3421_hcd->rx);
-		if (max3421_hcd->spi_thread)
+		if (!IS_ERR_OR_NULL(max3421_hcd->spi_thread))
 			kthread_stop(max3421_hcd->spi_thread);
 		usb_put_hcd(hcd);
 	}