parisc: led: Fix potential null-ptr-deref in start_task()

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

parisc: led: Fix potential null-ptr-deref in start_task()

start_task() calls create_singlethread_workqueue() and not checked the ret value, which may return NULL. And a null-ptr-deref may happen:

start_task() create_singlethread_workqueue() # failed, led_wq is NULL queue_delayed_work() queue_delayed_work_on() __queue_delayed_work() # warning here, but continue __queue_work() # access wq->flags, null-ptr-deref

Check the ret value and return -ENOMEM if it is NULL.