Threaded RTS: Fix a deadlock situation

The flag startingWorkerThread that is used by startSchedulerTaskIfNecessary
(in Schedule.c) has to be reset if startTask (in Task.c) decides not to
start another task after all (if a task is already waiting).
When the flag isn't reset, this leads to a deadlock the next time a new
worker thread is actually needed.

MERGE TO STABLE