FIX #7693: concurrent task execution: fixed dereference of NULL

[ProjectMutilation]

In the function TaskConcurrentCancelCheck() the pointer "task" was utilized after checking against NULL, which can lead to dereference of the null pointer. To avoid the problem, added a separate handling of the case when the pointer is null with an interruption of execution.

Fixes: 1f8675da4382f6e("nonblocking concurrent task execution via background workers")