scheduler: Fix bug in scheduling safety task

[prabakaranklst]

Parent Task-->spwans -----------> Task-A
           -->join_all
                                  Waker Data ptr ------> Task-B (External task, different type)
                                                         Waker Data ptr ----------------------------------> Parent Task

For the above scenario with safety worker enabled in runtime, after polling of Task-A which return error, it is detected as safety error and a safety waker is created from normal waker and wake() function is called. In the wake function, the waker data ptr is typecasted as (parent) task and further try to clone. But, there is no valid pointer in the task vtable for clone() and it results in segmentation fault.

As fix, a thread local safety error flag is set instead of creating safety waker. When the wake function is called, it goes through Task-B and then our wake() function is executed. In this wake(), the safety error flag is checked and the task is scheduled on either safety or normal worker.

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References

Closes #18

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