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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/1817c4b85011998604e5ff9a80a6e01adb7e7e81
• https://git.kernel.org/stable/c/a915c896f95a989a7759a73f8c064f5dc3775175
• https://git.kernel.org/stable/c/bd38395b901327f77a82112f006240de22cf2ceb
• https://git.kernel.org/stable/c/3db89bc6d973e2bcaa852f6409c98c228f39a926