fix segfault: parallel_do tries to acquire numThreads-1 threads. And there are cases it wanted to get 0 threads. And it was falsely returning back valid ticket because of missing conditions. Here how it was happening in real run: There are functions where nested threading can happen. So in segfault case (example:gather op) outer threading acquired all threads. and as inner functions tried to acquire threads themselves. And as the inner function was using faulty parallel_do it seg faulted.

Signed-off-by: AbdelRauf <rauf@konduit.ai>

libnd4j/include/execution/impl/ThreadPool.cpp

@@ -151,7 +151,7 @@ namespace samediff {
 
     Ticket* ThreadPool::tryAcquire(int numThreads) {
         //std::vector<BlockingQueue<CallableWithArguments*>*> queues;
-
+        if(numThreads<=0) return nullptr;
         Ticket *t = nullptr;
         // we check for threads availability first
         bool threaded = false;