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

In the Linux kernel, the following vulnerability has been resolved: 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.