staging: vchiq_arm: Fix possible NPR of keep-alive thread

In case vchiq_platform_conn_state_changed() is never called or fails before
driver removal, ka_thread won't be a valid pointer to a task_struct. So
do the necessary checks before calling kthread_stop to avoid a crash.

Fixes: 863a756 ("staging: vc04_services: vchiq_core: Stop kthreads on vchiq module unload")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Link: https://lore.kernel.org/r/20250309125014.37166-3-wahrenst@gmx.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: lategoodbye

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c

@@ -1422,7 +1422,8 @@ static void vchiq_remove(struct platform_device *pdev)
 	kthread_stop(mgmt->state.slot_handler_thread);
 
 	arm_state = vchiq_platform_get_arm_state(&mgmt->state);
-	kthread_stop(arm_state->ka_thread);
+	if (!IS_ERR_OR_NULL(arm_state->ka_thread))
+		kthread_stop(arm_state->ka_thread);
 }
 
 static struct platform_driver vchiq_driver = {