HPCC-15356 Possible race condition in ASyncFor can cause core in sem_post

Improvement following code review.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>

system/jlib/jthread.cpp

@@ -706,6 +706,7 @@ void CAsyncFor::For(unsigned num,unsigned maxatonce,bool abortFollowingException
         for (i=0;(i<num)&&(i<maxatonce);i++)
             ready.signal();
         IArrayOf<Thread> started;
+        started.ensure(num);
         for (i=0;i<num;i++) {
             ready.wait();
             if (abortFollowingException && e) break;